We put in two sinks because that's we wanted, and you should put in one sink because that's what YOU want. Frankly I am sad that people feel they need to design their bathroom fixtures around the unknown desires of mythical future buyers. I don't care whether a designer thinks either choice is a cardinal sin, or that without stone counters and a Toto toilet nobody will buy my house. If a potential future buyer doesn't buy an entire house because of something so minor they really don't want the house. To the reason for choosing 2 sinks in our bathroom -- before this house, dh and I hadn't shared a bathroom in 17 years. He used the bathroom in the finished lower level (raised ranch) near the garage/office, and I shared the main level bath with the kids. Now we have four bathrooms, each have our very own sink, and best of all nobody has to share with the teenage girl anymore. THAT was a hardship!

I live alone and have 9 ft. of cabinets in the master bath, with a sink in each end cabinet. It looks balanced and works nicely if guests at times use that bathroom. They then have a pristine second sink. In the big scheme of things, we're not talking about that much more money to add the second sink and faucet. And no, a 106" long vanity in my bath does not look too long, so 90" should look fine in yours. I would put a 27" sink cabinet at each end, with a 36" 3 drawer bank in the middle, for a 90" total. If you and DH are sharing the drawers, a middle drawer divider would take care of that. A 36" wide cabinet makes much better use of space than two 18" cabinets. Also, I love the luxury of having all that counter space in the middle to lay out clothes, or sometimes even for wrapping gifts.

The inspiration picture

The vanities are opposite each other in the master bath. The larger one is 8.5 feet from linen cabinet to wall and the other is 6 feet from shower knee wall to wall. Don't know if this is enough info but all I have at the time. Thanks for any input.