Typically mossy frogs are kept thirty gallon tanks that have a large water feature; anywhere from 1/2 to 2/3's of the horizontal space should be occupied by water. Milk frogs, however, do not require the large amount of water, and need taller tanks. This kind of ruins the split tank set-up. milks may need warmer temperatures as well, but I can't recall. If you had a +50 size tank you might be able to get away with a breeding pair of mossy frogs and a single milk frog, but don't take my word for it.
I personally am setting up a mossy vivarium; I've wanted them ever since I first laid eyes on them.