For me this is simple. Are you licenced to fly that particular aircraft as commander of that aircraft.
I know that if I fly a corporate jet with a guy who has an SIC he is flying from the right and we do an altitude bust its me who will carry the can not him.
The commander of the aircraft is just that if your in the right seat, something goes wrong and you carry the can then you are PIC. If you point the finger at the guy in the left seat and say my mistake BUT im only the FO he is the Captain then ???? There has to be a /US
Pace