I have heard that tree frogs can live at peace with each other. There are actually alot of people that do a mix in there tank. (Green Anoles, Different Types of tree frogs, Long Tail Grass Lizards, Day geckos can all live together to my understanding) I am not entierly sure about the Cuban Tree Frog with the Green Tree Frog but as long as they are arounf the same care I think it should be ok.
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Good luck!
P.S. As long as there is enough room for them both to have there own territory also.