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?
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What you are describing in scenario #2 is two toilets being common vented. This is most often fine as long and both toilets come off the stack at the same elevation. If you have one toilet dumping over another with the vent for both coming out of the top of the upper tee then you have one toilet wet venting another. This is a no go in every code I've ever heard of. The fitting you would use to make it right is called a cross (or a "double tee"). Check your local codes though as some jurisdictions wont allow crosses for toilets. In this case you would have to use a double wye with street 45s. This fitting arraingement takes up ALOT more room and sometimes is prohibitive.
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.