I am converting a single vanity to a double. Currently, a 2" drain/vent stack is off to the far left of the bathroom. The new double vanity will be on the right. What I think I will do, and please correct me if I am wrong is use a double fixture fitting in the middle of the 2 sinks, pipe the traps horizontally to that, with the drain going down, vent going up towards the existing 2" stack. I will connect the drain with a long radius TY and the vent with a vent tee.

I have a problem because I only have 2x4 studs and code only allows 60% boring or 2.1 inches (a 1-1/2" pipe). The smallest double fixture fitting I can find is a 2 x 2 x 1-1/2 x 1-1/2 (1-1/2 being the horizontals).

I am pretty sure I only need 1-1/2" for the drain and vent and 1-1/4" for the trap arms. Is it allowable to use those sizes for the pipes and use reducers/enlarger fittings to match the inlet/outlet sizes of the double fixture fitting?

I am in NJ and we follow the 2006 National Standard Plumbing Code if that helps.

Thank you