Well, here is where Darwinism would apply in my opinion. The reason that these specific examples of conjoined twins being more... long living, is due to human intervention and medicine. Because, if an animal is successful at living with a conjoined twin, they would then pass on their genes, resulting in more conjoined twins (I'm assuming it is genetic).The thing is, we don't see conjoined twins often in nature. The great majority of them die out. So theoretically, some stragglers may survive if the mutation isn't that much of a hindrance, but they would most likely not be overall successful. So they CAN survive, but it is highly unlikely.
