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?

Yes tie the vents together, 1-1/2 is fine, yes vents are required

A vent should never end in an attic, always must go through the roof.

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Originally Posted by Gary Swart

A vent should never end in an attic, always must go through the roof.

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Yeah, so the plan is to have all 3 of those vents tie together (2 will join in the bathroom from the sinks, and then that vent will join the original from the shower) and then have that one vent exit the roof.

heres a good setup on how to drain/vent dual sinks

You should consider using a double fixture tee...not a cross. Less fittings needed and only have to penetrate sill plate once. You will be fine connecting to existing venting as long as you are 6"above flood rim of highest served fixture.

4Tfever has the correct solution, as long as you use the back to back fixture fitting, (and NOT a cross, and NOT a doulbe combo), with a cleanout tee below it. Bring it up between the two sinks then use two 1 1/2" horizontal "arms" to run to the sink locations.

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Originally Posted by hj

4Tfever has the correct solution, as long as you use the back to back fixture fitting, (and NOT a cross, and NOT a doulbe combo), with a cleanout tee below it. Bring it up between the two sinks then use two 1 1/2" horizontal "arms" to run to the sink locations.

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So, is this what you're talking about?
And this is the fixture fitting?

Yes and Yes.

#1. Yes, but add a cleanout unless the pipe drops directly into the main line.
#2. Yes.

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Originally Posted by nola mike

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.

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Either will work, but I'm a fan of having the fewest possible roof penetrations (especially when one isn't reroofing soon or at the same time). Tying into cast stack in the roof might mean having to redo its roof pentration so it's perhaps half of one to 6 dozen of the other...or something like that. :)

I think you mean "six and a half of one and a dozen of the other".

Just maybe ... note the smiley! :P