In (UK) law it is the OPERATOR who specifies who the aircraft commander is. There can only be ONE commander on the a/c! So "Captains" can make up a crew but only ONE is the commander. Pilots must be qualified and current for the seat they occupy.
It has been my experience that sometimes Cabin Crew get a bit confused when two Captains fly together, especially when one is a Training Captain when the other Captain is the Commander. I used to inform them that whoever's name was in the "Commander" column at crew sign in was, by definition, THE commander!
In practice I dont think this is always as simple as some posters are making out. Let's say the operator nominates a "Captain" as the Commander on a flight and the "Second in Command" is a Training Captain. This, in my opinion, puts the TC in an invidious position if the Commander makes a decision affecting flight safety which he does not agree with. Crew members are legally obliged to obey lawful commands that the commander makes and indeed if they interfere might be liable!
My comments here are wrt to pukka Airline Ops. Perhaps when you are flying corporate for a small organisation where pilots know each other well the situation might be different.