Darts and retf's have different care requirements, first and foremost. Darts prefer much higher humidity than most tree frogs can handle. The 18x18x24 is a good size for 2 adult retf's. I personally, would NOT add any creature into a red eye tank with the exception of food. They are far to delicate to mess around with that.

Otherwise, frogs, due to a whole bunch of reasons that I am sure others will chime in with (I hate to hog all the fun lol), should be kept as single species tanks. Enjoy the beauty and the natural behaviors of your new little friends without stressing them out. Stress is one of the biggest killers of amphibians. If you want to keep a second species, a second tank should be in your future.