It is possible for a ship to have two or even three captains but only one of them has the supreme authority to command the ship. Yep, I experienced having three Captains, that's three master mariners on board and it usually happens on tanker ships.

Technically, only one person holds the legal title and ultimate authority of "Master" or "Captain" of a ship at any given moment. However, in the maritime world, it is very common for a vessel—especially a large cruise ship—to have multiple people with the rank of Captain on board. The second-in-command is often referred to as the Staff Captain (or Vice Captain). This individual typically holds a Master’s license and is fully qualified to take command of the ship if the primary Captain becomes incapacitated. While both may wear four stripes on their uniform and be addressed as "Captain" by guests, the legal responsibility for the safety of the crew, passengers, and vessel rests solely with the designated Master of the Ship. Having two high-ranking officers ensures that critical departments like safety, security, and hotel operations have executive-level oversight at all times.