Welcome to the Forum, Bethan. I am honestly feeling like I am the only member on here that does not have the joy of owning a White's treefrog. I may have to remedy that sooner rather than later...

It is the opinion of most of us on this wonderful site to not mix frog species, no matter how closely the species seem to be to each other or if they have similar care requirements. Each frog species has their own level of toxicity and though things might gel for a while, down the road, it may reach a level where both frogs will suffer. However, in that size tank, you could easily put 2 WTFs...and still have a nice sized tank for a White-lipped tree frog (hint hint!).