yes you can have two captains on a ship lok at the concordia there were two colonels on board at once and end run states the even if a officer outranks the captain of the ship they still have to follow his/her orders while on that ship
what happened to each vesuvius i know at least 1-2 are still operational but happened to the rest werent there like 5-6 i know the vesuvius was destroyed but what happened to the mt saint helens