If you want the sconces centered over the vanity, you need one that's 42" wide - that would give you 4" on the outside of each sconce. If the vanity is in a recessed installation, the balance would be really important, I think. But if it's just open space to one side of the vanity, maybe it could work. You could do a mockup to see if the 38" vanity would look really out-of-whack with the sconce placement.

:) that's exactly what i measured today and was going to tell my cabinet guy!

I think you'll want to buy two mirrors and three sconces. The balance will work out much better being able to have one above each sink. Furthermore, I would propose getting three lights as that will probably help with ensuring you have more evenly spread lighting to work with in the mirrors. I realize this bathroom vanity is longer but it does have the two mirrors and three lights. Definitely our preference when building.

If you do 4 sconces, 2 for each mirror, it will make each mirror seem its own entity, IMHO. If you do a double sconce in the middle, it seems like.....an after thought? And the sconces will not be equidistant from each other and the mirror. My OCD kicking in, I suppose. If the space between is larger than the mirrors, I think you should go with 2 separate sconces. If the space is smaller than the mirrors, I think you could go with a double light. Just make sure the spaces are fairly equidistant.