There is one captain on an aircraft. If things go wrong he wears it bigtime. Some captains are weak preferring to try to be a good friend all the time rather than being in charge. This is quickly sensed by a first officer who consciously or unconsciously will feel the captain is underconfident in his position of authority and consequently unreliable in an emergency situation. The first officer then feels empowered and will try to run the ship.
For instance the captain should be constantly watching the unfolding weather scene ahead on radar and from his experience decide which is the safest course of action. The operative words are "based upon his experience." While it may happen the F/O is PF for that leg, the decision which way to avoid threatening weather should not be a consensus. The captain should direct the F/O to take the avoiding action by giving him instructions on what heading and airspeed is needed. It should never be the case where the F/O is given a choice as this amounts to the captain abbrograting his responsibility. The F/O certainly may disagree and if the captain has commonsense he will reconsider his decision in light of his F/O's reasoned advice.
Too many times however the captain is inwardly afraid of "upsetting" the F/O and in the interest of maintaining perceived flight deck harmony, and under the guise of CRM decides to let the F/O dictate the course of action. That might be all very well if the F/O is in the LH seat undergoing command training. Once the captain resorts to weedling such as "Don't you think it might be a good idea to reduce speed?" or "Looks like some weather ahead - which way do you want to turn?" then he is asking for trouble. He becomes a friend and confidant instead of the captain running the show. There is no place for democracy on a flight deck.