This effect has been used in several feature films and one that really impressed me would be Coming to America, where Eddy Murphy plays a million roles and these roles interact with each other.
How best would you execute the effect.

Depends on the story board. In comming to america, most of the duality scenes were shot in such a manner that Eddie Murphys full features never appear on his dual character when they are both in camera at the same time. That could easily be acomplished with stunt doubles for the character whose features are obscured and with good continuity it will be perfect.

Lets skip to Jet-Li in "The One". Typical fight scene btw Jet-Li and himself, both faces showing, fuul bodied, in a factory, Lots of foreground and backgeroung elements. That is a challenge. Present a story board lets work with that.

Now I know i need to meet with you. What's your availability?
As for skipping to The One, I am sure they did some head replacement in it.

In that movie, during the fight when they had to show both faces, the second person wasn't Jet-li, I can't remember , but i think the head was digitally replaced as Sojiguns said, I think the other guy had a similar physical appearance.

On the other hand, How Best Would You Achieve The Effect Of Talking To Yourself (twin)?, you will be doing a lot of double shots plenty, plenty of compositing too, choreography
and expressions to relate to the twin character will be very important too.

I see quite a number of us have an idea as to how to execute the effect. I would say head replacement is a worthy try but involves too many procedures (colour correction, tracking, rotoscoping, matching lighting, etc)
I would stick to planning shots and ensuring we don't see both characters in the same shot too often.
By the way, what does everybody use to track? (2D tracking)

Well one way is to find a way of knocking of the background which in this case is not chroma. I will outline some steps below which will give you a good idea but please note that the result you get will depend a lot on how good your lighting is and on how proficiently you can handle your camera. The quality of the camera also counts. At the end of it all you still have to do a lot of polishing on the video (but for high budget videos it is well worth it).

Now your fundamental tool is the Image mate (I think difference matte in premier - my premier is not installed but I'll look it up). This tool works with an image and it will compare the image with the parent video and knock out all aspect of the parent video that matches with the picture (hue for hue, pixel for pixel). For this to work out right, the exposure and focus and lighting conditions of the mate picture must be the same as for the video(s).

Now make a good story board for each of the dual character and make sure you have gotten your continuity all figured out (it's not as easy as it sounds).

Set up the lighting on your set and position your cameras. (For each take, the cameras must be static, if the take must be such that the cameras move, try to shoot the video a little close-up and set focus on the subjects. The closeup shots will be normal chroma and you will later key it and replace it with an out-of-focus shot of the environment taken from approximately the same camera position and mimicking as much as possible the same camera track).[color=#990000][/color]

Take a still shot at a high resolution with the digital photo function of the camera or just record a few seconds of the set (you will only need one frame for your matte). Note that any object that is supposed to move in any way during the video shot must not be on set at this point in time. And any thing that is on set at that point in time is not suppose to move all through the shooting.

Now the video from this point onwards will depend a lot on the actors, your continuity, your directing and storyboarding. The movements of the dual character must of course be properly synchronised.  A good trick to use when you begin to have difficulties with your synchronisation is to use a different camera angle. This will distract viewers from possible lapses in continuity.
Each of the dual character will act his bit and go away. and you record it accordingly.

Back in your editing studio decide which one is the bottom video, you will not need to key that one unless where nescessary. Apply the image matte to key the top video so that the bottom one will show through thus displaying the bottom video in all areas except where the subject itself appears. It's just like chroma. The result is realy very neat on the first round so you will have to do alot of polishing up using your garbage mate and any other such compositing tools like After effects, of for ulead users Ulead video paint.

If you need to do frame by frame edit in some areas, export the shots as aif and edit in photoshop and reimport the sequence of aif files.

Back breaking routine (that is probably why it's not too common) but it works. Well that is the basics.

Enjoy.

@tomX, Nice breakdown. Nice!!!
But the image matte tool is really long journey in premiere. What of if you are dealing with images of different resolution? I would say using NLE software for such vfx work is not the best.
Another thing is for an action flick, static camera shots will be very few if there are any. Imagine you were given such footage to replace the head, how would you go about it?

sojioguns:

But the image matte tool is really long journey in premiere. What of if you are dealing with images of different resolution?
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Another thing is for an action flick, static camera shots will be very few if there are any

I probably didn't quite explain the matte thing properly.
If you need to take still shots it will be at the prepearation stage, last thing befor the actors come on set. (Don't forget that on a set where they know there will be animation and a lot of tricky editing, the animator/editor works hand in had with the set director all the way. And there is bound to be a lot of mark outs on the set to aid synchronisation of the actors movements etc.)
There is little likely hood of the picture for the matte being of a different resolution. don't forget all I said about your initial set up for your lighting, exposure, focus etc. You will also have to lock the exposure of the camera before taking the shot and during recording so that the camera does not keep adjusting its exposure. The camera also does not move. For a scenerio where the camera has to move then you will use a movable chroma board as a backdrop for the actors and, . . . see the quote bellow.

@tomX:

. . . if the take must be such that the cameras move, try to shoot the video a little close-up and set focus on the subjects. The closeup shots will be normal chroma and you will later key it and replace it with an out-of-focus shot of the environment taken from approximately the same camera position and mimicking as much as possible the same camera track. . .

sojioguns:

. . . Imagine you were given such footage to replace the head, how would you go about it?

I said something about doing a lot of polishing up afterward using other softwares like video paint etc. If you need to digitaly replace a head afterward then that would be a good time. However why would you go all the trouble to shoot the procedure described if you wanted to replace heads, you will simply have been shooting with a stunt double and then doing the head replacement thing.

Replacing a head digitaly on a motion video is also not that easy. Have you tried it? give us some insights.

For an action flick with moving shots that need head replacement, what I would do is reduce the head movement so tracking the head on the body is easier. Note that the 3D track would have been done to simulate the camera movement so a virtual 3D camera would be created and CG elements can easily be added. The tracking of the head would include using either native tracking in After effects or specialised tracking tools like syntheyes. When the tracking information is saved, any other head can be assigned the tracking information so it moves in relation to the body. The important thing is matching the lighting of both scenarios. Also colour grading is important to match both sources.
(601 Realtime is proudly the 1st professional colour grading facility in Nigeria. We started with Silicon Color Final TOuch, which was eventually bought by Apple and changed to Color).

As for matching the lighting, it cannot be perfect, even with a light meter. Like I have mentioned earlier, the video camera 'sees' images differently from the human eye. That is the essence of color grading in vfx.

For the record, head replacement has become the order of the day as long as the budget can accommodate it. Ricky Martin's video, Bend it like Beckham, etc.

i dont know why everybody's goin rocket science and no one has even bothered to mention the most fundamental method used for decades and will be around for decades more of achieving the clone fx. i mean has anyone ever heard of masking? . yes sometimes u gotta push the envelope but the fact is that simple masking and maybe a bit of tracking is enought to produce jaw dropping clone effect even in the biggest blockbusters. i dunno why the emphasis on head replacement. sometimes its the only way out but beleiive me in the jet li movie there was no head replacement. it was just clever masking and a stunt double. what they did was for places with both jet li's showing they simply masked to accurately syncronized composited footages. now let not forget. maskin in not just putting one over the other. in fact it sometimes gets more complex than the most cuttin edge effects because u have to animate the points , bezier's and tangents of the mask frame by frame when one overlaps the other or is meant to go behind the background footage. thats how u can get the two(or more)to interact or touch one another. in jet li's the one thats what they did. no hollywod studio want to just show of a tedious technique when they cud just mask and use stunt doubles for the other character when one is facing away or partially obscured or too far away or evein to brief for u to notice. there's almost no clone movie that i know where the two meet that doe'snt hav a stunt double. by cleverly editin the main actor on in both outfits or sides u wud b foold to thing he is 2 but there can be only one ( and thats me yu law!)even the fight scene in this" the one" is mostly stunt double only when the stand and face each other from the side is compositing used.its only when all these methods prove inadequate that head replacement is necessary.

I dont understand what you mean by masking. Please elaborate.

surely u r kiddin right? i mean seems to me that u re an after effects user and although i primarily use combustion i also use after effects and they all call it the same thing. MASKS. i dunno how u can miss it i mean doin all the compositin works u do. anyway its basically a tool u use to cut and trim certain parts of a footage like a duplicate footage for instance. u hav the original footage in the background and the the duplicate in the foreground. does'nt matter if its a 2d or 3d composite at this point. u use the maskin tool to cut out parts of the front footage say maybe the side edge of a tree which is a bit organic so u cant just use a standard crop tool to cut out its edge accurately. so u use the beziers or b slplines and cut out the video off the bark of the tree. by doin this u cud then maybe create the effect of a man commin out of a tree which he appears bigger than in width by puting either a keyed composite in the middle r even a raw video footage with identical camera angle with the man passin behind the tree and puttin it behind the maskd one. this masks can be animated too for moving object like a car or even a human. many times u want to even take a person out of a video footage like in lot of the montage's or breakers for these sport programmes where u cant get the athlete to come and shoot green screen so u can just use maskin to remove christiano ronaldo out of a football game in old trafford and place him in a 3d environment with the programme name or stuff like kpata kpata sports. im shure u get the drift now. i still cant believe u cud hav missed it. by the way even the head replacement itself is an extension of masking where u cut out the head or face of one face to another with syncronized movements and angles.

THat's what I thought. Because when I mentioned head replacement, it obviously involves rotoscoping (masking frame by frame) and tracking.