You cannot log P2 hours if it's a single-pilot aircraft.
Under JAA rules, the only time that two people on board a single pilot aircraft can both log the hours (meaningfully I mean, not SNY or something else silly that doesn't count for anything) is in the instructor/student or examiner/student situation.
Unless it's a single-pilot aircraft, but company SOP or other rules *require* a multi-crew operation. In that case multi-crew logging rules apply.