Mine have two different croaks so I am pretty sure the bigger one is a female and the smaller one is male. They talk to each other all the time! I had the female (Kiwi) first, and she never croaked. A few months later I got the smaller guy (Gingernsap) and the night I brought him home he croaked all night. They were in separate tanks, however. But I bet they could still hear one another, and after that night all they do is croak! It's so awesome. I have them in my bedroom and they only really start croaking when I go in there to go to bed. Since I got the O.K. from the vet that both frogs are healthy to put them in the same tank they have talked so much more
I love my little guys <3

Thanks for the quick responses. I am glad it is (possibly) a sign that they are happy.
I clean out their entire viv once every month, so they darn well better be!