Let Them Fight, Safely
It's difficult for kids to tap into their own conflict resolution skills if we break up every little argument for them. Instead of immediately intervening, stay within earshot and only step in if the disagreement becomes violent, if anyone turns to bullying, or if you feel the argument isn't going anywhere. Outside of that, you can act as a mediator to try and move things along, while still allowing your kids to reach an agreement on the issue at hand. Over time, they should be able to more easily get through their disagreements and get back to being friends.