It's good to talk!

Impose simple rules that are easy to apply

Talking is essential: they all need to say what's bothering them. If you feel that the argument is about a major conflict (violent language, severe distress), stage a discussion where you let first one and then the other explain how they feel. Each takes their turn so that the argument doesn't start up again.   

Let each speak in turn, ask each child what they think, encourage them to talk it over among themselves and take all the time you need to restore dialogue between them instead of fighting and yelling. If you get them used to discussing conflicts and talking things through, they'll come to understand that conflicts can be resolved in other ways than violence.