I think with 2 sinks you will have very little counter space if you are okay with that I guess 18” would work personally I would do 2 smaller vessel sinks to free up space or really just one sink, it’s funny in 52 yrs of marriage my husband and I have probably both been in the batrhoom brushing our teeth at the same time maybe 10 times so a double vanity is something we have never had. I like the bathroom to myself and so does he.