My bet on this is that it is possible, turtles that split apart many million years ago are still able to interbreed. Many different Cuora, Mauremys, Chinemys, Ocadia and Heosemys hibrids have appeared over the years, both in captivity and in the wild, just as example. However evidence shows that it is very unlikely you will acomplish this mating, common snappers and aligator snappers inhabit the same habitat in the states they both ocurr in. If this was an easy to produce hibrid at least some would have already surffaced like in the same way you see Graptemys, Pseudemys and Trachemys combos every now and then. Anatomic and behavior differences are surely the cause. Your best bet would be for you to get a HUGE male comon and put it alongside a sexually mature smaller size ali snapper, but even then it may never happen. I would also be very suspicious of anyone claming to have such hibrid, hard evidence is needed.