Haha, I've seen frogs do this many times. Have you heard the normal one sing on a regular basis, or just when the other's on top? They only sing when sexually mature, they have no reason to otherwise. It would appear to me like you may have two males though. Horny males looking for some action will sometimes hop on whatever frog happens by, even if that other frog is a male. The "singing" in return is more of a "Hey, get off me!" than anything else. Weak, chirpy singing isn't really singing. When these frogs sing, you'll know it. They'll just sit there belting out sounds and inflating their throats.