When you have two children of roughly the same age, fights are bound to happen once in a while and it's typically very hard to know who started it. Here is how I handle it: they both get to go through a "timne-out" period, i.e. spend 10-15 minutes to cool down, either in their rooms or better, in the batherroom. A friend once saw me doing this and was surprised that they would not keep on fighting in the bathroom. Well... I guess it is because all of a sudden they forgot why they were fighting and they focused all their attention on what they needed to do to be allowed out of the bathroom...