I have had many check outs where the instructor has specifically told me that it will be quite in order for me to log the flight as P1 for myself and that he or she will be quite happy, and indeed intend to, just sit there for all intents and purposes, as just passengers. (unless they feel have to intervene) Under these circumstances I have naturally logged the flight as P1, since this is what the pilots agreed to be the case at the outset of the flight. Whether the instructors concerned then also logged the time as P1 for themselves I have absolutely no idea, but it has certainly occurred to me that they may well have done so, either deliberately or unconsciously.
I do not wish to cock a snook at authorty or undermine regulations with a purpose, but in the scenario under discussion where two qualified pilots are flying in an aircraft, and in the event that for whatever reason they both logged P1 time ...then ...does it really matter? does any one actually care...will any one ever find out ?