For the record, I managed. Just quoting myself to make it more visible to the future visitors:
I don’t know why Yabee exported four joints. Maybe because there are two vertex-groups, and two joints, therefore 2x2=4? I had two identical “door_left” joints and two identical “door_right” joints, so I just removed the duplicates with a text editor.
Important note:
When I joined my two doors into one single mesh, I first did it with the doors in closed position. That created problems: Panda realized that some vertices were at the same place, and it simplified the geometry, fusing the two half-doors together. Because of that, opening the doors resulted in a rubbery surface stretching from one door to the other as they opened. Yuk.
To solve that problem, I made the rest (default) position of the door opened: panda can not simplify that geometry since there is no overlap between the two doors.