User blog comment:Sprinklemist/Sprink Talks About Stuff/@comment-1808106-20110630213521/@comment-1874924-20110702032412

As far as I know, it's physically impossible for fraternal twins to be conjoined (Siamese). They have to be identical because of the way this defect arises, and because fraternal twins (which come from separate eggs) have separate amniotic sacs, placentas, etc.. and so have no physical contact with each other during development. This means, among other things, that both must be of the same sex and both heads will look the same, with the possible exception of things like hairstyle.