You should act as a mediator between them. Try and figure out why they are fighting and help them the best way you can. You should however be careful not to take any side as you might either make it worse or create an enmity between you and them. If you can't, look for someone else who can help you out.
it's not your fight if you don't want to be in the middle of it when get yourself out of it. tell them that you aren't picking sides and would like it if they understood your feelings and respect you enough to not put you in the middle