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I am updating my post to say "I did put in 2 sinks" and each day, when I use the bathroom, I dislike it. Every time I comment internally, "I hate the 2 sinks". It is such a small area, and not to have any room to put hair dryer, curling iron, make-up, etc. I have come to realize, for me/daughters, that counter space is a necessity in a "working bathroom". There are never 2 people in there at once brushing teeth/washing their face. The board, across the sink, still exists and it works perfect. I am only sorry that I put in two sinks as now, I live with a board. (big sad face)

I think you are right on the edge as far as width to have two separate lights. We would typically do one center light (3-4 light) with a 6' vanity with a single mirror running the entire distance. If you had two separate mirrors, the two light fixtures work quite well. That being said, it's done all of the time with two separate lights over a single mirror. The fact that you have lighting over each of the sinks via the can lights, IMO I would lean toward doing a center set light.

6" is a very common height off the floor. And it continues in 3" increments with 15" being the highest as plumbers tend to have issues after that.