There have been a number of colubrid snakes that have been crossed in captivity that are not even in the same genus. Kingsnakes have been bred to cornsnakes and bullsnakes and so on. Some of the offspring have even proved to be fertile which throws the taxonomists into a frenzy, questioning everything they thought they knew.
Although your two turtles are not subspecies of one another, they are in the same genus.
My guess is that they could produce viable eggs.