New toilet plumbing

[MICk]

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?

[Redwood]

You may not wet vent between floors.
If that pipe serves as a vent for the first floor toilet you cannot use it.

[devidmike]

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.

[jnaas2]

There is a reason as redwood stated that you dont use a vent for fixtures upstairs and one of them is when you flush the toilet upstairs it will more then likely pull or push air in or out of a fixture downstairs which will lead to sewer gas in you house. There is a reason for all the codes and as stated you dont need to follow them if you are not having it inspected but its sometimes easier to do it right the first time verses doing it again, the choice is yours