Harvard Business Review:

It’s always uncomfortable when two coworkers fight.

If the conflict is getting in the way of teamwork, talk to them. Let them vent to you, and try to get both sides of the story. Show that you understand how hard the situation is, and then explain how their fighting is affecting the team. Maybe say, “You two not getting along is hard for everyone, and it’s preventing us from doing good work.”

But before you offer advice, ask if they want your help. If they do, you can problem-solve together. It’s rarely a good idea to involve the fighting coworkers’ boss – unless the problem is truly hurting work – because that would escalate the situation. If the issue is outside of your comfort zone, suggest they turn to a third party, like someone from HR.