I have two boys 13 and 12, They fight over everything. Sometimes coming to blows. I have tried just about every trick in the book, often wanting to cry myself, because nothing makes any difference (yes single parent!)
When I re-read how to make friends and influence people I tried to think what would make them WANT to stop. And have some success with rewarding the one who was willing to stop first. I ask one of them to 'Be the better person' and if I can get one to stop responding to the other, the argument is over and I make a fuss and try and reward that one. Next time the other one wants to be the better person.
Occasionally it works!
Poppy