When my girls were young and got into arguments that they couldn't resolve by themselves quickly, I would hand them both a brush and Ajax household cleanser. They knew what that meant: they had to go to their bathroom and scrub the shower and toilet TOGETHER until it passed my inspection OR until they were friends again.
Very quickly, they had to become a united front against Mom. They knew their cleaning job would never pass my inspection, so they HAD to make up and be friends. If they were stubborn and took longer to makeup, I still got a cleaner bathroom out of it!
I have to say, they always came out giggling, and before I knew it, they always worked out any differences quickly, because they didn't want to see me coming into the room with my brush and Ajax.