There are two different ways that gerbils fight. Play fighting and Serious fighting. Play fighting would be chasing each other around and jumping on each other and things like that. Serious fighting would involve blood, scratches, etc. You can tell if it is serious because the gerbils will be losing weight, have bite marks (especially on the tail), and stop sleeping in the nest. Watch them for a little and make sure that everything is okay. If blood is drawn, seperate them because it si unlikely they will ever like each other again. Good Luck! I hope it is just play, that's what it sounds like, but i can't tell for sure.
