I have my bathroom sink rough plumbing coming from the 3" stack vent, running about 2 feet. My question is: can I use that same drain line for another sink?
Reasoning: about 2 feet from that sink drain I have a wet bar sink I want to put in. Using the same drain line sounds easier, only because of where the stack is, not because of putting in the second trap.
Consider all the dual-basin kitchen sinks... "If it happens, it must be possible."
Speaking from personal experience, only use one trap in a dual sink bathroom. With two traps, you can get air pockets blocking one or the other's drain. You also promote stinky slime growth inside the pipe between the two traps. I don't know the biology with that.
As far as I know, having two separate sinks served by a single trap is not code compliant. You will need to replace the existing sanitary tee on the stack (inside of the wall) with a double sanitary tee.
Normally, these double tees are used for adjoining bathrooms, so you can have a sink on each side of the wall, and share the same stack. In this case, you would be installing the tee parallel with the wall, instead of perpendicular. You will need to extend each side of the tee to reach where you want the trap to be for each respective sink. In order to do this, you will need to cut a short piece of 3" PVC, and then put on a 90 degree elbow and attach another piece of PVC so it will come out of the wall. Then you can attach the trap adapter for each sink.
My double kitchen sink was installed (licensed plumber) as the picture, and seems to work fine (10+ years). I see no problems.
What the code says: If the drain in the two sinks is more than 30" apart, code states that they each need their own trap.
What reality says: Sometimes having two separate traps sharing the same tie in can be an issue venting wise. Negative pressure issues can be solved by adding AAVs or AAV traps, but positive pressure issues will be a problem with two traps.
