IIRC the 2-seater was more correctly called a 2-holer. Butt I might be mistaken. One size for children and one size for adults.
Back home we had the 2-holer. I often wondered where the carpenter got the dimensions for the holes. My 24" carpenter's square (80+ yrs.old) has 100s of graduations for everything one could possibly need but nothing for butts. I never figured out why a cresent shaped moon was sometimes cut into the door either.
One thing was for certain - no one in their right mind would have had an indoor commode (thunder mug). Doing natures calling inside where one slept, cooked and ate was unthinkable! How times have changed.
Fred