I would not suggest interbreeding different species of turtles. Many times the hatchlings will have problems. Why create turtles with deformaties? Also many times the eggs wont even hatch. Last but not least, theres no market for them. If you want to breed, stick to the same species.
-----
Currently in the US Navy so I have sold off my collection. But over the years I have had dozens of turtles, fish, geckos, and other lizards. Cant wait to get back in the hobby once im out though!