
The best solution are special units or differentiated jurisdictions. An Immortal Flame Captain is on a special unit dedicated to butchering the Amalj'aa and Ifrit, while a Flame Sargeant is on a more mundane protection of refugees arriving to Ul'dahn territory. Tecause they are on different "fronts", so to speak, you can succesfully argue that the Captain has no authority over the Sargeant.
It would be akin to one being a police man from one city while the other, while also a policeman, is from another different city. Unless their mutual higher up (who is an off-screen NPC) actually organizes them in a hierarchy, they don't have any business commanding each other around. You can apply this to anything up to Marshal, Admiral, General or whatever.
Something that has to be kept in mind when roleplaying officers is that in-character ranks should come with responsability. In-game we rank up as fast as a speeding Ferrari and the game doesn't put any real responsability on you. But in-character those ranks shouldn't be achieved in a matter of days (unless you want to use one of your 'special things' card on that), and they should carry some responsabilities besides the obvious privileges of commanding other people around. Something as simple as the illusion of not having enough time can send the message that your character is actually busy being an officer.
It would be akin to one being a police man from one city while the other, while also a policeman, is from another different city. Unless their mutual higher up (who is an off-screen NPC) actually organizes them in a hierarchy, they don't have any business commanding each other around. You can apply this to anything up to Marshal, Admiral, General or whatever.
Something that has to be kept in mind when roleplaying officers is that in-character ranks should come with responsability. In-game we rank up as fast as a speeding Ferrari and the game doesn't put any real responsability on you. But in-character those ranks shouldn't be achieved in a matter of days (unless you want to use one of your 'special things' card on that), and they should carry some responsabilities besides the obvious privileges of commanding other people around. Something as simple as the illusion of not having enough time can send the message that your character is actually busy being an officer.