the reason why there is only one rear foglamp is that so it won't be confused with brake lights.
if there are 2 rear foglamps, the trailing driver will have to observe your car for a few seconds more to come to the conclusion that you are not on the brakes.
the convention is to place the rear foglamp on the driver's side.
i think it is a necessity in heavy rain or thick fog, to reduce your chances of being rear-ended. My 1989 190e does not have the wiring for the rear foglamp, and i wish they left it there - it's just a single wire!