do you just say, "stop!" or "cut it out!" - i have wanted to say that before. the truth is, they won't be that happy if you just yell. also, your friends are facing a hard time. they also want to stop fighting. try to calm them down. remember in chapter 1 where i said tips to calm down. tell them some of these tips.
you may have had it up to here. perhaps you could talk to one, or both, of your friends and explain that they need help. tell them that you are going to ask a school counselor about it. they are there to help you. soon, if you can't help anymore, try to tell an adult. they can SURELY help on that.