I would never do it. I even keep all my different species of box turtles separate. I'm not real sure, but it has always been in the back of my mind that different species wouldn't interbreed. For example, anything in the "geochelone" FAMILY would probably interbreed, but anything outside that family would NOT breed with geochelone. Different tortoises within the Manouria group WOULD interbreed, but Manouria would NOT breed with Gopherus...and so on.