Jules is right, is sounds as if they are just acting out normal ratty behaviour. As long as no blood is being drawn then you're ok.
I think you're doing the right things, introducing them outside of their cages, and letting them visit the other one's cage to get used to the scent.
Have you looked at links on intros?
I don't think you've got anything to worry about. It might not be playfighting exactly that you're seeing, but just the struggle to establish dominance that all rats go through before settling into a heirarchy.
Good luck!
