As stated, I think it is open to interpretation. I believe that at a very minimum, there has to be a rated pilot in the right seat, UNLESS a rated CFI is flying the left seat.
Therefore, to take it one step further, one COULD deduce that without a CFI in either seat, ie just rated pilots, that the person in the right seat would be the PIC. Again there is a difference between "logging" and "acting as" PIC.
Maybe the big question is this: Who would be the PIC, in the eyes of the FAA or insurance company should there be a mishap, or violation? There was such an argument many years ago, namely the "Walkup" decision. I am not a lawyer, (yet anyway), and I do not have the decision in front of me, but it hinged on who was to be violated when an aircraft violated airspace. From memory, there were 2 rated pilots on board, one had thousands of hours and an ATP, the other was a low time CFI. It was determined that, because instruction was being given, the CFI was the PIC and therefore liable for the "actions" of the aircraft.
Maybe, flying lawyer could enlighten us?