Here is a trick i have learned, when the fighting is at thier worst tie thier hands togather, and let them figure it out on thier own, make it clear that they won't be intied until they can work togather for at least an hour. And everytime they start to go at it, retie them. Sounds a little odd and maybe to some cruel but I have found it really works, for one they don't like it, and for 2 they are forced to work thing and out and problem solve on thier own. I did this with my older 4 out of desperation and was surprised by the results, I even did it to them at one of my sons soccer games, that time it was humiliation that stopped thier endless berage of insults and just plain meaness, other parents got a kick out of it thats for sure! LOL!
I also did it to my 6 and 3 year old last week, and got a good day out of it, all I have to do is ask them if they want to be stuck togather again and they stop......usually. Just a different spin on an age old problem. Good luck!
I see that some might find it cruel, but for future info i don't use duct tape or anything demeaning, I use a sweat band for the wrist, and my kids actually enjoy it, they like to see what they can do using only one hand each, and since most of their fighting is over video games they have learned that that is the one thing they can't do when only using one hand, when their time is up they have forgotten what they were arguing about and we usually have a good day. I didn't say it would work for every child, every child is different and sometimes you just have to get creative in how you do it. The world doesn't work because of a bunch of "me" people it succeeds because we have to learn to work as a team.