Yep, I think the others are right. If the plane doesn't requires 2 pilots, and for your average PPL it's unlikely you'll be flying one that does, then only one person can log the time - the PIC is P1.
The other guy, no matter what license he holds or what he does during the flight, is a passenger. I think you could agree to share the flying, so one of you is P1 for half the flight and the other is P1 for the second, but the only way 2 people can log the time is the situation SKYYACHT mentions where one is an instructor - in that case he is P1 and the other pilot is PUT.
I had this conversation with someone who was recording his time as P2 when he was in the right hand seat and doing the radio, navigating etc., but that is not allowed.