E.J.
There's not much information to go on here, but perhaps teaching them to problem solve and what to do in conflict could maybe help?
Example: Susie takes toy. Sally learns to talk about the problem and verbally ask it back without hitting Susie. etc.
Maybe they just need more tools to learn to compromise or to talk about the problem.
This is a vague answer since we don't know their ages or what specifically fighting is. IE verbal, physical, name calling etc.
Good luck!