Proper way to drain/vent dual vanities?
I'm putting in double sinks in my new bathroom. Original plan was to plumb the drains and supplies through the floor, but it seems like the correct way to do this is to stub out through the wall. So I'm planning on installing 2 drains that will drop through the sill plate, wye into 1 pipe and then join the main 3" stack. I was originally under the impression that they didn't need to be individually vented (as long as there was a vent within 10 feet of the drains). Doesn't look like that's the case though, and that these sinks need their own vents (please correct me and make my life easier if I'm wrong on this). I could put AAVs on them I guess, but it doesn't look like it will be THAT much harder to do it right. Right now, there is a 1 1/2" vent from my shower that terminates in my attic, and the main 2" cast vent that goes through the roof. I have to relocate the 1.5" vent anyway. Here are the questions:
1) Can the 2 sinks share 1 vent line? I think the answer is "yes", but the horizontal run needs to be 6 inches above the spill line for the sink. If that's the case, I'll have to cut through a couple of more studs, but save drilling through the sole plate--it's also easier to make those connections in the bathroom than attic.
2) Can I tie this new vent into the existing shower vent? Is 1.5" the correct size?
3) Should I then route this through the roof or tie it in to my existing cast vent? I think it would probably be easier to route it through the roof, but I'm not sure.
Anything I'm missing?