I am always baffled seeing mother-in-law and wife turn into enemies whenever they stay under same roof. Really I don't get it. Shouldn't they be friends?

Why should they stay under the same roof. It is the same with any adult situation especially when the role of each one is not clear.

@ Outstrip

In some countries, it is mandatory to stay with your in-laws. A larger chunk of South East Asians don't date, parents choose their partners and the women HAVE to stay with the man's family, extended family included. Same goes for some arab communities as well.

@ Topic
Ever heard the old adage "familiarity breeds contempt" ?
They should definitely stay together, but not long enough to become hostile.

Even here in Nigeria, some men bring their sick mothers or fathers from the village to stay with them in the Cities, either until they recover or until they pass on (i.e if no one to care for them). It may not be acceptable to most women, but the couples concerned should discuss it and come to an agreement. Some MIL's are nice although some are not. But whatever happens treat them as you would your mother. If she remains hostile to you even with all your love & care, your husband & his family will not condone that. Know that should you cooperate with your husband, it will earn you his love & respect. Also know that should the situation be the reverse, i.e. your mum or dad needing such assistance, your husband will cooperate with you as well. Sometimes these situations arise & we must cooperate for harmony sake.

only two occassions
1- omugwo ie if my wife's mum doesnot know how to do it
2- celebration/festive period,which can be managed 4 2 weeks!

apart from that its better they pretend to be friends from afar or u are dead

Do they have to pretend to like each other? I believe a woman can have a mother-daughter relationship with her MIL if she goes into marriage without preconceived notions about the MIL.

My mum has a great relationship with her DILs. They all treat me as their own sister and vice versa. But I agree that in any of such relationships, too much of familiarity breeds contempt. Good fences they say, breed good neighbours.

MIL and my mum cannot stay with me for more than 1 month at a stretch,exceptions are Omugwo and health issues.

I am sorry but this things never work out mother in-law and daughter in-law,i for one will not accept that cos most mother inlaws tend to take over the home and cause problems ,so its a no no.,they are allowed to visit but not moved in and the longest visit shouldnt be more than 2 weeks .

I don't think Mother-in-laws should stay long. Two weeks max!! That's tolerable. Whether the mother-in-law is nice or not. Let your son live his life!!