Hi

When you queue for Duty Finder, you can change job in between to play with while you wait, but you will have to change back to the job you chose for the Duty Finder when it starts.

Remember thought that you can't change jobs while in a fight, so if you're fighting something while in another job than the one chosen for the Duty Finder, you will have to run away to change jobs (and if you can't, you might miss the Duty, so careful !)