I had wondered about this in the past, and it appears to be true: fraternal twins can have different fathers. When fraternal twins result from two seperate uh... incidents... it's called "superfecundation." And then to really screw with your mind, there is such a thing as "half-identical" twins, where the egg kind of splits before it is fertilized and then two little spermies come along and geeeeee is it all funny and complicated. I'm not even going to mention Chimerism, which I watched something about on TV a bit ago.

twinning facts