This is my first post on Permies. After I read the posts about 2 holers, I felt compelled to share photos of this one, a 18 hole outhouse I encountered in rural Finland. It's at a school that's still in use but the outhouse hadn't been used for who knows how long.
In the photo, you can only see 4 seats, there are another two to the right just out of view. The red thing on the left is a urinal. Sharing the same large pit are another 12 holes on the other side of the building, which are divided into 3 rooms. In earlier times, all the goodies fell into the pit and then would be shoveled out when needed. For me, it was a moving experience to stand in the pit, look up at all the holes, and think about how it must have been when they all 18 holes were in use.
When I was there in August, the pit was amazingly clean and was being used for storage. We set containers under 3 of the holes so it could be used as a composting toilet during a festival. Along the front edge of these three holes, we added a common urine diverter. The diverter was made from some scrap metal formed like a rain gutter.