Why does this work?

If one of the boys is pitching a fit, and then the other boy starts to pitch a fit, I tell him, "You can't have a fit.  We can only have one boy at a time being naughty.  You need to wait until he is done to have your tantrum."  Oddly, this works.  Usually the second tantrum never happens, but a few times the second boy has actually waited for his brother to finish having a fit and then started in on his own.  Go figure.