Window Washing

The four upper windows in my office extend about 12 feet up from the floor; so I’m not surprised when the window cleaner hauls a ladder in and climbs up to wash them. I leave for a few minutes, come back, and there’s a different cleaner washing the lower four windows. I ask one cleaner why they divide the labor, and he has no answer.
Does having two workers raise the average product? It could if they take turns and share the tedium and danger, but I don’t see why, since the first worker told me that he always does the ladder work/high windows. Does just having a colleague reduce boredom and perhaps raise productivity? Perhaps they take turns driving the truck from place to place? Overall, a strange way of organizing production.