It is never advised to mix species... not only do frogs secrete toxins harmful to other frogs and creatures. You need to match there temp and humidity levels, which are different with many frogs. Then there's the size factor. Whites are huge and veracious eaters and can or will try to eat anything remotely smaller.

The only 2 frogs that I have heard having almost the same care needs are Greys and American Greens. Assuming you get a large Grey and a Small Green to balance the size issue. But it is still not recommended due to the toxins that a stressed frog may release.