Each species has very different behavior. A leopard frog would bully smaller frogs like wood frogs and be bullied by larger frogs like bullfrogs and green frogs. It doesn't matter if you feed them in separate containers, they will still cause stress for each other. It isn't really the fact that the frogs can eat each other as the fact that the frogs think they can. If you must get another frog, I would strongly advise on getting another leopard frog and upgrading the tank size. The new frog should be quarantined for at least a month and a fecal test should be performed. Furthermore, the frogs should not have a size different more than a quarter of their nose to vent length.

I can guarantee you that even in tanks run by the most experienced people, mixed species tanks will not yield the same long lifespan as same species tanks would