Question.

[Louis Charles Bruckner]

I will agree that the enclosure is getting a bit crowded.

ready made terrariums can be expensive especially tall ones for tree frogs.
If you are handy and don't mind making our own.
it is not really too difficult.

you can make a quite nice tall terrarium out of a 20g long.

just set it up on one end and get a pice of plexy cut to block off the bottom 1/3 of the tank and another pice to cover up
the rest of the open area.

you can get plastic hinges online and make a plexi glass door.
I would get a grate or some kind of template to drill 1/8 in holes in a neat pattern
at both the top and the bottom 3 inches.
or you can just cut out a window and silicone glue a non metallic screen over it.
what ever looks good for you.
if you drill holes just use a 1/4 inch drill pit in your hand and use it to smooth out the
holes front and back
Make sure to use a new drill bit that is sharp that will give you the best finish.
Use 1/4 in thick plexi.

also don't for get to put in a drain at the bottom.
Drill the plexi with a 1/8 in bit and then use a 1/4 in and use air line. as your drain hose.
If you have plexi left over I would cut two pieces that are 1/2 the with of the tank plus 1/4 in to make raised area on both sides of the drain tube so the water will flow down to the tube.

only put enough silicone to seal all the edges of the plexi.
or water will collect under the plexi and putrefy. yuk!

all you need now is some fine gravel to cover the drain tube 2in then larger gravel layer another 2in then some expanded clay bio balls. some non metallic screen over the bio balls and add eco earth and you are ready to add plants. and then the inhabitants.

Get aquarium background mat to cover at least the back of the tank.
I would do two sides.
I am sure you know what to do for lights and heat and moisture.

[Louis Charles Bruckner]

On and there is a bit of a fly in the ointment with this photo.

[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.