Now tell me why you are not P1?
Because I have to defer to your decision as to whether you will take command away from me as and when you see fit? Thus, I am ultimately not in command but you are?
Anyway, don't drag me into the discussion like this. I'm all for private pilots getting checked out by other private pilots and being+logging P1. I'm just pointing out some of the legal and practical problems with this.
But to be honest, the other way around has legal and practical issues too. Supposed I'm being checked out on an aircraft by another PPL (the owner, say). If I'm not P1, then the owner is, and that means that he has to pay at least an equal share of the costs. I don't think there are that many owner-pilots willing to pay for checking out other pilots...
As others have said, this has been done to death here and I've seen all the arguments from both sides to defend both positions. And the only solution I've seen is to explicitly agree beforehand who will be PIC and what the other persons role will be and what he will log, if anything.