I have a neighbor that wants a new double-bowl vanity installed in place of a single one. The current drainline comes out of the wall from the left side of the cabinet. The new vanity will sit away from the wall on the left side (there will be a gap), so I will have to move the drainline to the back of the unit and install a separate drain for each side, as the cabinet has drawers in the middle that leave a gap of only 1/2" space in the back. If I notch the studs in the corner, will I need something other than nailer plates to cover the pipes for support of the studs? Also, will I need to install an in-the-wall vent over each drain line since the actual drains will be approx. 3-4 ft. from the exist vent? Is notching the studs the only way to run a drain around a corner?

You can use existing drain / vent set up with modifications. Install 2" x 1 1/2" Cross San "T" and run run two separate pipes, one to each sink. ( in your case, one run will be shorter, the other longer ). We don't notch studs but rather we drill. As per Code, notching studs will weaken their structural integrity. Drilling is accepted. Yes, use nail plates at each stud where pipe penetrates it. As far as water goes: you can use Double Angle Stops or run h/c water supplies with the drain.

Enclosed is a photo of typical, two sink rough-in...

I am relocating sink from one wall to the other, so I will be going through studs 90 degrees from each other. Won't this be too tight to bore through to install a sweep ell? Being that I will be approx. 6' from exist vent line, what does code say about the vent? Also, can't use dual valves as these are separate sinks due to the drawers in the middle of the cabinet. Also, never cared for dual valves, I like individual controls.

How about a picture or sketch? There are limits in venting/drain locations and it depends on your area's code and sizing of pipe, etc. Is this a basement/or slab installation or different?

Agree with CyberHeater: photo or sketch would be very helpful to give you the correct advice

It's an upstairs bathroom. I'm thinking of cutting the existing tee out, then installing a sweep tee, rotating it 90 degrees or towards the other wall. From the existing (plumbing) wall, it appears as though I have enough room to bore through double studs, then install a sweep 90 ell. From here, I'll use 2-45 degree ells to rise up approx. 6" to the new sink. For the first sink, I'll use a sweep tee, the proceed to the other sink and come out with a sweep ell, rising 1/4" per foot from start to finish. Code is 1/4" per foot, correct? Would like to get some pictures to show what I have, will work on it. Cabinet that I'm installing is built like a tank, and weighs as much as one too. We're talking 3/4" cherry wood throughout with a granite top. The 2 bowls are china, but were glued underneath, making replacing them almost impossible without replacing the whole top as well... which was glued down to the cabinet as well.

As I understand it, I can notch studs if I use a Simpson stud shoe, per code, or is this no longer true?