I think the misnoma here is PIC U/S. This is a 'get out' term used for the benefit of the First Officers traning with respect to being brought up to speed to pass a command course.
The AOC holder MUST nominate the aircraft commander and will specify the requirements laid down to do so.
If you do not meet the AOC holders requirements, i.e. You haven't passed the command course, you are Co-Pilot and therefore NOT PIC. If the AOC holder considers all pilots to be equal and all to hold the rank of PIC then a designated Commander for each individual flight must be nominated, the other pilot become the FO and thus Second in Command.
The FO, no matter if holding the same rating, licence or with more experience and hours, is not the PIC and can only log PIC if the Captain croaks during the flight.
One Commander, who will allow PIC U/S to be logged only if they want to and the company rules allow.