I have a female Flying and a male Crested gecko in the same 20 gallon aquarium. They seem to be getting along fine. Hiding in trees next to each other. I know their caresheets look the same. Am I not supposed to be keeping them in the same enviromnent ?

Please don't do this. Their care is nearly the same but it's not a good idea to mix different species for several reasons. They are from different parts of the world, one may carry bacteria thats not harmful to its species but potentially harmful to the other, they eventually will probably fight, when the crested grows larger it will probably attack the flying gecko.

Even though crested geckos are communal, before putting a new crested in a tank with other cresteds, you should quanantine your new crested to make sure nothng is wrong with it. This means keeping it in its on seperate tank for a while to observe its behavior, eating habits, physical state, and its droppings. If you introduce one crested that has a sort of disease, it may mean the end for all your geckos. Now why would you want to introduce other species of geckos together?

Main thing is that both may die just because theyre different species and contain different bacteria. No offense but please for the sake of your geckos seperate them. Yes maybe they will get along fine and nothing at all will happen, BUT (now im going to make an asumption here) youre kinda new with cresteds. I think the best thing would be to seperate them.

Good luck and keep us updated.