The first thing to understand as parents is that as much as children fight to win over the sibling, they also do so to gain parental attention. So don’t mediate. You will always be accused of taking sides no matter how fair you try to be in your judgment.
Encourage them to find a solution on their own. As a first step you could separate them and take away the reason for the conflict (so for instance taking away the toy they both want OR switching off the TV on which they both want to watch their favorite channel). Let them cool off and understand that fighting will help none of them. Tell them to work out a solution on how to get the toy back and how they will play with it without fighting (for instance they may come back saying that they will play with it one after the other).
Negotiation is also a good technique to use (See case study with this topic)