Perhaps I misunderstood. Do you mean the rear doors of the bus, or of the plane?
At any rate, both doors in the COBUS are used at either end (terminal and at the plane) and the L1 and L2 doors are used - with separate Aviramps - on the widebody aircraft.
Upon arrival, J class deplaned using the front L1 door into their bus which leaves first. Y class uses the second L2 door until the J cabin is empty, and then both doors are used. Upon departure, the J bus arrives first, followed by the Y buses. Economy passengers use the front L1 door for seats F-K and the L2 door for seats A-E. Ground staff hold signs and direct passengers as they step off the bus and it seems to flow smoothly. I’m not sure what the procession is like for the J passengers seated on the starboard side of the airplane.

Every RSO arrival and departure is timed at every stage in the quest for continuous improvement.