It's recommended that you quarantine any new frog for about a month to see if it shows any sign of diseases. (I kept mine in a large clear tupperware container with mesh hot glued over a hole in the top.)

Maybe you could gradually introduce them by putting the containers nearby and then only leaving the frogs together for a few hours at a time. Are golden tree frogs territorial? I'm used to White's which aren't at all. If they aren't, you shouldn't have anything to worry about and slowly introducing the frogs is probably not even necessary. Do make sure that they're roughly the same size though.