In the "airline" world, the captain is always in control and the F/O is second in command. It is decided who flies the aircraft and who does the communication prior to take-off. There could be two captains on the flight deck, but one is designated the left seat and one would be designated the right, the F/O ( copilot ) seat.
Sometimes the F/O is flying and the "left seat" is doing the communication, sometimes it is the opposite. In an emergency, the captain will hopefully take over control and then is in charge. At a "bad" LH landing attempt at EHAM the captain took over from the apparently inexperienced female F/O. There a hundreds of threads on many, many websites for outsiders, like regular passengers, to get an inside to this kind of operations ( pprune.org is one example ). If you are lucky and fly on UA mainline and have a cooperative pilot, who is not adverse to broadcast Ch9, you will hear, who is the captain and who will fly the plane today.
I have never heard of any married couple, or even "friendly but not (yet) married" couple in all my time inside and outside the aviation industry. I, personally, do not agree with this scheduling, although I wish the couple stays together forever being in the airline profession.