The first wizard can change the future. He stands poised, ready to rewrite the future so the other attacking wizard does not exist there.
The second wizard can change the past. He is ready to make the other never born.
This doesn't change anything, both of them know that in that cataclysmic moment they will irrevocably remove both of themselves from the equation of time itself.
So, in the eyes of mutually assured destruction, a compromise is made.
They never met. And they'll never meet again.