Planning a Double Vanity That Works
Width Is the First Constraint
Two bowls in a run narrower than about sixty inches leaves each person a strip of counter and no useful landing space between them. Below that width a single bowl with a long counter and a second mirror often serves better, and it costs less to build and to plumb.
Drawers Beat Doors
Under a basin, cupboard space is a dark box with a trap in the middle of it. Drawers notched around the waste give the same volume in a form that can actually be used, and the difference in daily experience is larger than any finish choice made above the counter.
Two Bowls Means Two of Everything
Each basin needs its own supply pair, its own trap and its own connection to the branch drain. That is more work inside the cabinet, more penetrations through the back panel, and a longer horizontal run that has to keep its fall. It is routine work, but it is not free and it should be quoted.
Counter Height and Who Uses It
Standard vanity height suits nobody in particular. Comfort height is taller and easier for most adults, and it is the wrong choice in a bathroom used by young children. This is worth deciding on the people in the house rather than on what the showroom has in stock.
Lighting Above and Beside
A single fitting above a mirror throws shadows down a face, which is precisely the wrong effect for the tasks performed there. Vertical fittings either side at roughly eye level light a face evenly. With two positions that means four fittings, and the wiring for them has to be set before the wall closes.