I know that this may not be the correct answer you were looking for, BUT,
When I am being ' checked out ' by an instructor, on an aircraft and flight that I am LEGALLY allowed to make, I log it as P1......end of.
I dont care wether he or she is an ATPL Concorde captain or the King of Siam, I am not on test, nor am I being given instruction, the person in the other seat is mearly a passenger, albeit a knowledgeable one.
Regards
Jepp