The way we do it (not to say it’s better or worse than any other way), is that all the physical handling exercises, such as max xwinds, RTOs, EFATOs, etc. have all been trained, practiced and signed off for competency before the line training starts. The aspiring commander is expected to act like one and the command check is a formality where all they need to do is demonstrate a safe and commercially aware operation - the decision that they should get four stripes has already been taken. The line training is really to emphasise and bring out the soft skills in a real environment with lots of people to manage.
The aircraft we’re talking about here I guess are certified for 2-crew minimum (or more if you need an Eng and/or Nav). Everyones’ licences are endorsed for multi-crew operation for commercial ops: you can’t go one down and therefore input from both sides of the cockpit is not only desirable, it is necessary in order to comply with regulation. There will always be a possibility that someone in either seat makes an error or omission and it is beholden on the other crew member(s) to point this out and/or do something about it if required - this can happen at any time, not just in training.
When you sit in the LHS (or RHS as a trainer) you have to give a certain level of trust that important things will be done right, but be prepared to intervene if they are not. If you go down the road of not allowing others to take part in critical decision making, one day you will find you make mistakes too, but nobody else is empowered to help. If we are taking about certified multi-crew aircraft, the captain has responsibility but so do the other crew. If there was an opportunity for another crew member to give input that would have saved the day, but they didn’t, they would be under investigation almost as much as the captain because it is their professional responsibility to do so.