having control is taught from lesson 1. Even under instruction, the student will have control unless the instructor TAKES control after saying : I have control!
All pilots are taught to be clear about who has control at any time during the flight.
Any sensible pilot would also do the same towards who is P1 prior to the flight. It is called human resources management and is as important as choosing the correct runway. Any pilot (in any control seat) participating in any flight whilst leaving the P1 issue unclear is actively encouraging a dangerous situation.
Even when pilots "share activities of a flight (one the controls, the other navigation or radio), the fact that P1 relinquished radio or nav to a secondary pilot does not relieve him from total responsibility as P1