If we're in the car and the kids start screaming, I pull over and tell them I'm not moving until they are done. I can't concentrate on my driving when they are screaming and hitting each other in the backseat. Usually they stop immediately and I say "So you're done?" and get back on the road. If they go at it again, I pull over again. I think if you're consistent about this, and consistent about separating them in other places (or having a consequence), they will get the message after a few days that the bickering and negativity is not going to be tolerated. I know that's not the main issue, though...you want them to stop feeling so negative about each other about everything and like each other.
