The outhouses that have two holes in them are designed for both kids and adults to use.
An adult has a bigger sized butt than a kid does so the two holes would be different sizes to allow either a child or an adult to use the outhouse without falling into the outhouse holes.
Also some outhouses had the same size holes in them to allow more room to distribute the waste when a lot more people were using those outhouses.
The two holes in the outhouse weren't for two people to use at the same time although they could if they actually needed too.