humping question

[Fisher]

My two gerbils which I "thought" and was told were both females have been humping each other all night.  I really wouldn't be surprised if one was a male but why would they both be jumping on each others back.  It's not just one of them.  It is pretty much equal with each one humping the other.  Can they really both be females or is this totally out of the question now?   Sara Fisher Baltimore, MD