Hi there Gnag
I really wouldnt worry about the two frogs being together,(if one is larger than i would worry), but they both come from the same area in eastern north america and they coexist with eachother in ponds lakes and slow moving rivers. A 20 to 30 gallon should be enough for the green frog. If Gnag starts to bully Bumpy a large steralite tub would work if you need to separate them, just add 3-4 inches of water, a dry spot for basking and a hide, and i almost forgot, A LID

Good luck with your frogs