Surely the solution is for the 'check out' pilot to be P1 and the pilot being checked to log nothing, that way there is no ambiguity at all about who is in command.
I know many don't seem to agree with DFC on this issue; it seems that's because in practice having 'two' PICs works fine, but it would just take one crazy prospective group member to disagree with the 'check' pilot over something, refuse to hand over control and then accuse the 'check' pilot of attempting a hijack. Ambiguity over who is the real captain can cause accidents.