When two PPLs fly together in a Cherokee, or an instructor flies with a student in a C152, it is a multi crew environment. You have two people who can potentially grab for the throttle, have their feet on the brakes, look up frequencies or hear whether the controller said "left" or "right". There must be a designated PIC, and he should dictate any procedures on which there may be disagreement. That procedure may be, "sit there and don't do anything unless I ask you." He shouldn't have a yelling fit if the other pilot forgets, but the other pilot shouldn't forget. (Flight instructors are allowed to have yelling fits, if they are doing it on purpose, believing that what that student needs at that moment is an intense experience to promote learning. Sometimes one "are you trying to kill us??!!" is more effective than twenty repetitions of "airspeed" or "gear down").
A&C you are so right about "folklore" procedures. "Why is that?" and "Where can I learn more about that?" are good questions to separate the legends from the good ideas.