I'm doing it. I have a tiny powder room and two would look too heavy. I'm putting it above the mirror. Haven't hung it yet.

Can your medicine cabinet be recessed at all? If you are down to the studs, now is the time to be framing out the recess for the cabinet, cut the studs box it out with header. You say the vanity has a side wall on the left side. Open up that wall, create more storage there with an additional, preferably recessed, medicine cabinet. Think about the projection of the lighting fixture, particularly if you go with over the mirror mount. You want to try to get the light coming out beyond the projection of the cabinet, this avoid the cabinet creating a shadow. Projection is a factor, as is the distance above the mirror you can install, the shape of the light, the directionality of the light, etc. With a 48" vanity I think a 30 - 36" mirror with a sconce on both sides and a second side wall cabinet would be great.

"... How many would be the next question..." And the answer is...wait for it..."it depends!" :-) Depends on so many things, beginning with the design and type of sconce: is the light throw down, up, sideways or all of the above, or none of the above. There are many, many kinds of sconces, just as with any other type of fixture. At the end of the day, one is left with a question or two: 1) Am I concerned about lighting something(s) specific, and; 2) Do I care if the fixture is visible, and/or is an "eye magnet"? The answers to these questions will tend to answer "how many?" :-)

I really need the 2 sinks after 30 years. Respectfully, duplicate sinks are a want -- not a need -- and no matter how much you want them, you simply don't have the space. You're rationalizing /justifying why it'll work, but numbers don't lie -- maybe that's why so many people hate math; it points out glaring truths. Is it possible to "bump out" the bathroom a bit? I strongly suggest you get a bunch of cardboard boxes and create a "mock up" so you can see the size. Incidentally, I've been married 28 years, and I see repetitive sinks as a negative.