Looks like the old P1/s argument will start again.
P1 means you are in command and wholly responsible for the aeroplane and it's safety.
The commonly held view is that other than that you can only be P u/t in an aeroplane that does not require 2 pilots.
Another commonly held view (amongst some very senior instructors I have flown with) is that with their superior knowledge and qualifications - even if they are practically a passenger - there name will feature first in the accident report so they never in real terms relinquish command and you must book P1/S - which is good enough to add up as P1 for the revalidation hours.
I have never had it questioned at revalidation time.
In the USA they have clarified this position and there are times when you can (I am reliably informed) BOTH claim the P1 hours.
(should I wear a hard hat or just get my coat).
