I managed to at least reduce the fighting by using the method described in "Siblings without rivalry." I was amazed at how well this worked:
Basically you catch them being good--watch for moments when they're getting along nicely and comment on it. And ignore it when they fight as long as they're not actually doing damage. When you truly can't stand it--banish them until they settle their differences. You can either make them take the fight outside or to another room, or separate them and send them to separate rooms.
The point is to reward the behavior you want and extinguish the one you don't want. Since even a negative reaction is reinforcing--any attention is better than none--it does help.
Relegate that old crockpot for dying and get one of the newer ones -- some are even programmable. You will love it. . .Happy passover. I make really good matzoh balls btw (I use olive oil in them instead of margarine or chicken fat).