A double vanity looks like a fair division of territory. Real mornings are less symmetrical. One person needs a basin for two minutes; the other needs somewhere to put six things while finding the seventh.
Watch the overlap
For a few ordinary mornings, notice when both people actually need running water. One couple asked us for two basins because getting ready felt crowded. Their overlap turned out to be a hair dryer, a makeup bag, and somebody reaching across both for toothpaste. We laid those items on the proposed counter. A second basin occupied the one clear patch they were trying to gain.
If the overlap is real, two basins can be worth the space. Draw them at their actual size with the taps, countertop edges, and nearby wall. Two people should be able to stand comfortably, not merely fit into a symmetrical elevation.
Open every drawer on the drawing
Ask for the internal cabinet arrangement with both traps and supplies shown. In that bathroom, we settled on one basin with a clear preparation area beside it, but first moved a proposed drawer away from the plumbing route. Don't count a long drawer front as a long usable drawer. Show where tall bottles stand, where the dryer goes once cool, and whether either person's drawer blocks the other person's knees.
Kohler’s vanity installation guidance illustrates why plumbing and cabinet installation must be coordinated. Use the chosen vanity and basin specifications for the real layout.
An off-centre sink can be a useful answer
One basin placed to one side can leave a generous preparation surface, if the selected countertop and cabinet arrangement allow it. That is our design suggestion, not a universal prescription. Choose two sinks when the daily overlap earns them. Choose one when the second basin would mostly hold things you wanted to put on the counter.