The OP's question was relating to two qualified pilots flying the aircraft so any limitation on solo flying is irrelevant. The Aircraft Commander may sit in either seat (subject to his competence and any local rules such as flying club orders or Ops Manuals).
I sent a pilot (an instructor) to fly a TB10 solo in the right seat last week after flying a type familiarization flight with him; he was, of course PIC on that flight!
I flew two flights today as Pilot in Command and instructor, the student and I swapped seats for the second flight because the training course he is undertaking required him to experience flying the aeroplane from both seats.
There is no restriction requiring the instructor to be able to operate all the controls from whichever seat he occupies. When I flew the second flight today I was sitting in a seat from which I could not reach many of the critical controls. In this case you had better be sure that the pilot in the other seat fully understands how to operate the controls which you can not reach yourself!!
If there were any statement in a POH saying "the pilot in command must occupy ..." then that would have to be respected but I have never seen such a statement.
Last word; if you are going to fly from an unfamiliar seat make sure that you know how to operate all the controls from that seat and that you can handle all normal and non-normal procedures from there. Don't just do it without training!
Happy landings
3 Point