Aside from your earlier post...............
IMO - Student pilot (no licence, no permission to fly supervised solo) on a dual training exercise cannot be PIC. Auth sheet will detail FI as P1. Why does a bum on a seat make a difference? IMO it doesn't. Suggesting that an FI only becomes P1 when he joins the aircraft is unmanageable to my mind. FI signs auth sheet accepting the aircraft and from that point onwards the aircraft is their responsibility.
In a two crew environment the handling pilot is not necessarily the aircraft commander. Who would you say was responsible then?