Amazing feature. He put an additional toilet next to the existing toilet The toilets are back to back using the same waste drain. The room next to the toilet was a closet. He just tapped into the same drainpipe. I image if you flushed both toilets at the same time you might have a problem sharing the same drainpipe Have you ever heard of this technique?

Very common, particularly in apartments, condos, schools, hotels and in some houses I have seen. Unless there is a blockage, you would not have a drain issue unless there is no vent which would surprise me.

Yes they share a common vent. Marvelous

Easy way to add a toilet and no traps needed since the toilet is self trapping.

As said above, it is a common installation to have 2 toilets connected back to back. You see it more often in commercial installations where bathrooms are designed back to back and share drains, vents and water in one plumbing wall. There is a special fitting to accommodate this type of installation. Also, pipe will absorb simultaneous flush from two toilets, no problem.

Milo

Very interesting