According to the job list, the Tinkering skill increases the speed of composting (and presumably operating other machines as well). Therefore, I set my skilled tinker to be the only one allowed to compost. However, other dupes would also clean out the outhouse when it was full. I assume this is unintended behavior.
Set dupes to not be allowed to compost. When they need the outhouse and it's full, they will empty it anyway.