my powder room will be right below my bathroom on the floor above. there is a vent pipe coming into the bathroom from the roof and i was gonna tie into it to discharge the bathroom sink and bath tub. then i was gonna use the same pipe for the powder room sink to get vent and also discharge. so the portion from the roof to the bathroom would be vent only but from there to the powder room sink it would perform both vent (for the powder room) and discharge (for the bathroom above). below it would be no more ties into only flow into the main in the basement below. i was wondering if this dual use for both vent and drain in that portion is OK. if it is a problem, i can run double pipe in the portion so that the vent and drain don't mix but obviously, the former design would be simpler.