Question.

[kueluck]

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.

[DVirginiana]

Don't house them together... Even though the care is similar, they have different behaviors and produce different toxins that will stress the other frog, especially in a small environment like a tank. I've never seen any scenario where housing any two kinds of amphibian (or reptile for that matter) is recommended.