I doubt that humans will ever get the label of 2 headed like other animals but it is geneticaly possible. I wish I understood the genetic lingo but there is a difference between conjoined twins and additional limbs/heads even in humans. Snakes can be conjoined twins or simply have a genetic abnormality that formed a second head, tail or whatever else. Same with all other animals. If humans can have extra toes, fingers, arms and legs and not be because of a conjoined twin why not a second head?

I agree with kikakiba, I think they are wonderful and beautiful creatures. Having one and keeping it alive to a ripe old age would be a great feeling.