They aren't a species they are just anomolies, I think I read somewhere that it is the equivelent to siamese twins, and that they just share the same body. They tend not to live long in the wild, I've heard of two headed snakes dying becuse they desperately try to go different ways and the thin skin between the heads split - not overly bright.
Two headed turtles are not a species as moray said. I have read several debates on the sale of 2 headed turtles and it's a sad subject. Some people have and do sell them for higher dollar amounts than normal (1 headed turtles) and that is sad to me. They don't often live long and suffer from many different internal problems and to try to make $$$ off that makes me sad!