If the First Officer cannot take his role for some reason, and the company only has another Captain available, can two captains fly the airliner, assuming they routinely fly as captains for that company in the aircraft of that type? Can one of them simply take a FO role for that flight?
In other words, is the First Officer a separate certification that must be current?
Yes.
What allows you as a pilot to fly large aircraft (legally under the FAA) is a type rating. As long as the two Captains hold proper type ratings for the aircraft they suffice for the required two crew members needed to fly the plane. However one of the two of them will be performing the duties of the first officer and that should be decided prior to the flight.
First Officer and Captain are rankings given by the airline not any governing body. Both the FO and Captain cary the type cert required to fly the aircraft (as far as I know).
The limited (and quickly dwindling) exception to this is aircraft that require a crew of 3 where the third is a flight engineer. The Flight Engineer position does hold its own certification.
Two-captain pairings are possible, and not particularly exceptional even -- obviously, both captains are fully type rated in the aircraft, so together they make a full crew for the aircraft (barring a F/E as Dave points out).
There are two scenarios where this is common:
A pilot is being upgraded to Captain of a specific type or is otherwise undergoing training "on the line", such as differences training. He will occupy the left seat, while the instructor Captain will occupy the right seat during his line upgrade flying.
A line check is being performed. In this case, the line captain occupies the left seat, while the check captain occupies the right seat.
There is also the case where while you have a captain and a first officer, the first officer has more seniority/experience than the captain. This is common with a junior captain -- "green on green" rules prohibit pairing a junior captain with a junior first officer, so many times, a junior captain will fly with senior first officers until they get some seasoning of their own under their belt.
Yes if the captain is right seat checked - in my oufit anyway. At my previous employer all capatain were approved, in my current one just management and trainers. Makes for easier scheduling perhaps but requires a bit more sim time for the right seat V1 cut check.
