When I fly with someone I stress to them that I want them to alert me to anything that they think is wrong, that they disagree with and/or that is causing them concern.
I also retain the "I have control/You have control" that I learnt in lesson 1 of my PPL. If they made any adjustments to power or flight control inputs without asking first (for lack of better phrasing) I would immediately be telling them that I was controlling and that they should draw my attention to it but not act themselves, and if they continued to do this I would be considering not flying with them again.
2 people controlling an aircraft when neither knows who is in control is a very bad thing whether it is two PPLs in a 152 or a commercial crew in a jet (look at the 447 accident).