Chuck, I'll definately agree about it not being a problem letting someone have a go whilst you are in flight, but I have benn asked the question on a number of occasions and I'm struggling to find in our myriad of over complex rules whether it is actually allowed or not.
It looks like Mike has answered it, but I'm still not totally convinced about it being within the 'rules' as such. Mind you it's a pretty unenforceable one if it is disallowed!!