No FO should assume command, but be willing to intervene when absolutely necessary due to ensure critical safety and be confident enough to advocate their point of view. My duty is to hear all inputs from the team, keep an open mind and praise where praise is due.
Peeps need to get away from "My Command, Your command"and "My leg, your leg" ,mentality. On my flightdeck - its OUR leg/task, but ultimately, as skipper, legal responsibility of my crew, pax, freight, aircraft and those I fly over is mine.
Before I ever intervene in the LHS, first question, is this flight safety critical, or is it rapidly deteriorating into a flight safety critical issue?
Second, is it economic? Will try and elicit info rather than "tell" the other seat, as no-one likes having their thought train pulled apart. This is where in depth knowledge of the operation is essential. Case in point, recent departure from mid-west airfield, transatlantic, squalls and plagues of frogs. I had been keeping a close eye on FR24 as well as the online departure board and it was obvious we would not get a clean breakaway with 30+ holding aircraft and about 20 delays stacked. FO wanted to take PLOG. Rather than telling them what to take, I carefully posed the scenario and let him come up with the answer. If done in the right way, the FO will appreciate your honesty and tutelage. He certainly did and asked for further D+G into other scenarios he was interested in. The bottom line for LH, PLOG is not necessarily cost efficient if you run the risk of diverting and dont have the duty to get into destination within a reasonable time frame of connections.
Ive operated with Co-captains before,I usually leave it to the first leg debrief, give them some humble pie but then firmly re-enforce my responsibilities, and theirs and respectfully request that in the name of professionalism, we need to ensure there is no doubt over the correct cockpit gradient. These people can rapidly turn a bad situation into a dangerous one, particularly when their egos out cash their ability and respect of the commander.