Can an interior wall be built over an existing heat/air vent allowing air in both rooms
You want to make a closet in your master bedroom , to me that is a non load bearing wall(s). This won’t be hard you just cut the bottom plate where the vent hits and move it up a foot or so. Then a wall register on each side provides the same amount of heat or possible slightly more flow because of less restriction.
I put vents in large closets to help keep air movement and reduce the chance of mold and mildew in the closet. You don’t have to duct up to the vents as these are interior walls but I would seal that short stud bay at the bottom and top to keep the air directed in the bedroom closet area.