I'm also converting a single to double vanity in Illinois (I can hear the groans ). My situation is bit different, we had a right hand sink and after opening the wall all of the plumbing is coming off the right side so I can't use Terry's famous diagram. I'm not too worried about the water supply it's tapping into that giant stack! How do you add another Wye into that?
Much help is appreciated. Unrelated but based on everyone's recommendation, I've ordered a Toto Drake II for the ladies of my house. I'm hoping it can flush Coke cans!
(in the pic below I just illustrated the Hot/Cold and where the copper terminates)