If you're gonna house them together (which I don't agree with) you'll need the 36 x 18 x 36. The White-Lipped Tree Frog (litoria infrafrenata) get huge!! They call it the “giant tree frog” for a reason. I'm sure you want to keep them healthy and happy so the more space, the better with these big guys. I have 3 WTF in the 36 x 18 x 36 and they use every bit of space. I know because every morning I'm cleaning the glass.