I was talking about this with my barber. He said that in school he was pretty good and got called over a lot to finish shaves that others had started before the victim passed out from loss of blood. He pointed out that you don't have any of the feedback that you get when you're on both sides of the blade.

The closest I've been to shaving someone else is someone else's arm, and I'll tell you, I got the blade about 1mm away, and making the first touch I almost chickened out because I realized I wouldn't have any feedback at all. I can't imagine doing someone else's face.