Its actually an operation manual combined with an agreement with the CAA via your ops inspector which decides how the hours are logged.
If you have written your SOP's and trained for a dual crew environment and the aircraft is deemed suitably equipped to run dual crew ops you will be allowed to log it as dual crew.
Examples of this are the loganair twin otters on the western isles and the Highland airways F406 fisheries patrol aircraft.
If you don't have the approval the only person who can log anything is the PIC. If there is an additional pilot onboard to comply with public transport rules about SPA IFR ops and autopilots or should I say lack of autopilot. They don't get to log anything unless the PIC is a CRI/CRE on type and then they can log dual on empty sectors.
Its actually a bit of a money saver for a first job for the pilot. Because it is a SPA aircraft you don't need a MCC course to do the type rating. Then after 500 hours multi crew ops you get an exemption on the requirement to have done the MCC if you then go on to do a multicrew type rating.
Just to add it is not certain that you will be given the right to log multicrew time, the fisheries are an example some of the fisheries operators (current and past) are/were allowed to log multicrew time. Others were not and that was using exactly the same airframes.