Deebrief used his keyboard to write :
Place a long strip of metal in the track. That blocks the door from
sliding. That will not interfere with the other door. Remove the
metal when you want the right door to slide.
If you want to prevent the guests from removing the metal strip, drill
a hole in it and screw the metal downward through the bottom of the
track.
The metal strip could be a trimmed bit of aluminum sold in the hardware
store as flashing (a strip of metal used to overhang the eave of the
roof so that no water can get under the shingles.)
McGyver