Originally Posted by Mr.Phillips
That's exactly what I'm talking about. But if they're sharing the same drain, what about sewer gas?
Each fixture will still have its own trap. Of course the trap for the toilet drain is built into the toilet.
You can "stack vent" the fixtures, meaning if you can run the drain straight down and the vent straight up, you can dump both waste arms into a single fitting called (at least here in the northeast) an "Estabrook" fitting. I'm sure it's called something else in other regions, but the technical description is a 3" Tee Wye w/2" Side Inlet.
In this configuration, the two fixtures share a common drain and a common vent stack. You can even tie your lavatory waste arm into the same vent.
You can individually vent each waste arm and then dump each one into a common soil stack or branch drain.
Or you can do a "wet vent" wherein the waste arm for the shower also vents the toilet.
It all depends on the layout of the bathroom and what the framing will give you for choices.