There's too many quotes to cut/paste - but I'm not sure I agree with the "Who is *doing* what" principle. I tend to subscribe to the "Who is *responsible* to the authorities for whatever was done".
This brings us right back to "Who is the commander of the aircraft?".
The "You do the radios, I operate the controls" does *sound* a little like PPL's playing at being airline pilots, especially in a single crew aircraft - what difference does it make?
If, for example, left seat physically flies it, right seat does the radio - then right seat tells left seat that you are cleared to enter airspace, or climb to Flight Level nose bleed - but you susbsequently get reported for a level bust or airspace infringement - who will carry the can?
If 100 hour handling pilot suffers an engine failure, sets up perfectly for a field to land in, but 1000 hours "passenger" says, "Hmm, that field over there was better" and 100 hour pilot goes for it - then misses both......who is responsible?
I flew with a guy a couple of months ago. As I *always* do in the particular scenario we were in, I offered him the choice. He elected to be the commander. During a subsequent, genuine, undercarriage failure (Piper Arrow), he left the traffic pattern of the airfield we were at to investigate.
During his investigation, the aircraft climbed into controlled airspace without a clearance. Had we been reported, it would have been his neck on the line. Before you bite me, I was not in a position to help him with a warning.
"Who is *RESPONSIBLE* to the authorities?", is the question that I pose to all my licenced acquaintances.