An excellent topic.
A friend and I fly together frequently and have a couple of standing rules.
1. If either of us asks for control the other relinquishes the controls immediately.
2. We are not shy about mentioning items the other could improve upon. It's an opportunity for us to improve our skills.
You might not mention it to him, but really I'm the better pilot. Even though he has more ratings and is more proficient than me. I just tell him he's better to keep his ego inflated.