yeah, if i had both species i wouldn't too, I'm looking for why's and can't think of anything yet. usually the reasons for not mixing species are different geographic area, thus different husbandry requirements, possibly becoming food for each other, different toxins.
I was thinking genetically close species can live together and even have offsprings, provided of course the enclosure if big enough, say different king snakes species can interbreed, you can have F2 and so on, so why not african bullfrogs.