Question

Question!

Answer 1

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.

Answer 2

That all depends on the environment that they live in, if they are living in the city there is a chance that they can but the chances are slim of course since providing for say so may kittens/puppies and having that extra head could cause that side to starve.
if that side passes away it could also kill off the next head due to extra weight it may not be able to carry around.  
there are no real studies or statistics on it but i can tell you from what i know it would be much harder for them to live without care from say us or if there was a small litter of pups and kitties.  really all depends.