By the end of the day, they have finished the refactoring, created three new classes, and tested them. They have a couple more refactorings to go before they can integrate, so they version their code and agree to pick up there tomorrow. We switch pairs as often as practically possible, don't let a bad apple spoil the whole barrel. That's a subtle way to remind people of what they're not supposed to be doing. It usually does the trick.
Some people claim that pairing is two people doing the work of one, so it doubles development cost.
Two people working together can do amazing things that a maverick can't. Well, not if people are dedicated to what they're doing. XP takes discipline. We are focused. Besides, sometimes the distractions are helpful.