Another thing is size. I understand that Milks get to about four inches at the biggest and White's get to about five, let's say your White's is three inches and your Milk is one or one and a half inches? White's will eat the Milky. That's the first thing that popped into my mind when I read this questions, besides the toxins.

Sorry it didn't work out. D:

Does anyone know if he could build a barrier between the two frogs if it would be alright? As long as the barrier was secure and the frogs couldn't see each other regularly? It's just a thought if you still wanted to use your viv for two species.