I'm not sure what to tell you about the width of the mirror. But I have just done a bathroom that has sconces on either side of a mirrored medicine cabinet. The space between the cabinet and the wall is 9" on each side. An average sconce needs about 5" in width, leaving only 2" on each side. I think it would look crowded to put a sconce in a much smaller space. To me, 5.5 would be small.

We have looked hard, VERY hard! I see nothing that is 2 inches wide, and only one that is 3 inches and that has an LED "module", so the bulb could never be changed, if need be. My husband doesn't want something like that. I don't think the very narrow ones come with changeable bulbs. I've concluded that in order to have sconces on either side of two mirrors, I think our counter would need to be longer than 74". In order to have three sconces, our cabinet would need to be different. I'm encouraged that m_gabriel came to the same conclusion with a similar situation. Makes me think I'm not off base.

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.

Thank you so much for these really helpful comments. @kandrewspa, Interesting suggestion about changing the sconces next to the mirrors, and I'll consider it although I do really love the ones I've got. Re the ceiling fixture, I do have one but unfortunately it is primarily an exhaust fan with just a small LED light, which I forgot to turn on when I took my photos. I need that fan, and there isn't room for an additional ceiling fixture. That's why I wanted to put in the lighting over the mirrors, to augment the sconces at the sides. I do already have both categories of lighting on dimmer switches; forgot to mention that. @JustMe, for up-facing sconces to go above the mirrors, do you mean something like this or this from Justice Design, or this one from Capital Lighting, or this one from Swiss Passport? Would you place them above each mirror, or between them?