I am adding an upstairs master bath in a small room connected to the master bedroom. There is a wet-wall containing plumbing for an existing upstairs bath and existing downstairs bath. The existing upstairs and downstairs toilets each have their own wastepipes to the main drain in the crawlspace. Can a new toilet connection be added to the vented vertical wastewater pipe that serves the downstaires toilet or would this be a no-no? If not, can a 2-way toilet arrangement (one on each side of the wet-wall) be installed on the vented upstairs vertical wastepipe?

Linkback: https://www.plumbers.nz/q-and-a-toilets-wc-bidets/62/new-toilet-plumbing/251/

If you want to stay compliant to your building codes, no. If you don't care about code violations (may be an issue if you ever sell your house) then go ahead, it will work just fine. Just remember the plumbers 3 important rules: Brown trout don't swim upstream, don't bite your fingernails and payday is on Friday.