re: Calling plumbers of TD

You can absolutely turn a single vanity into a double. There are multiple options with the water lines. Is copper stubbed out of your wall? Pex? If copper is present-you can remove the individual shutoffs and install a dual outlet 3/8x1/2 compression shut off valve; or you can sweat a tee on the bull of the fitting and install individual lines and shutoffs to each vanity. If there is pex-do as described above and install tee.
The drain is simple-remove old trap adapter and install a wye fitting. Run individual drain lines to each one with the proper pitch.