Well, this one is rather difficult to explain or imagine!!
According to the story on Atlas Obscura, there are two possibilities for this two-story outhouse in Cedar Lake, Michigan. The first is that this odd outhouse used to be connected to a hotel or boarding house. Before a fire destroyed the building, a footbridge once provided access to both floors of the detached restroom.
The other theory is that the original owner, purportedly William Nelson, wanted privacy for his wife and daughters from the lumberjacks and other customers who used to frequent his store. So he built the two story structure with 8 seats (read: holes), four on each floor.
All I wish is that there was an inside look so I can see how the mechanics of this operation worked, because I know what a regular outhouse looked like, so it’s hard to imagine the same thing going on above you! My thought is that the top floor must have carried the, ahem, “contents” through a chute behind the person on the lower level. Still, if I were a lady back in those times, I would try to time my “trips” for when the lower floor was empty of customers if at all possible!!