Oh yes, it doubles the light! Be sure to choose sconces with more than one bulb, and put them on a dimmer. That way you should be able to get as many variations of lighting as you'd need. I had the very same setting in the dressing room in my last house, but I don't have a picture that shows it that way anymore.

This is is the dressing room I'm talking about:

I don't have a picture, but it used to have a vanity all the way across, and mirrors above that covering the wall. On the mirrors we mounted two sconces, each with two candle bulbs. There was plenty of light. (We changed it our for resale; some people feel a wall of mirrors looks dated, and the vanity we had was dated, so we redid it).

That's a very handsome vanity, mtnrdredux. Can you tell me where it's from, or was it custom?

And yes, it does make it much brighter if the lights are reflected.

Ditto what the previous posters said; i have two Lurgarno (from Restoration Hardware) sconces on my large mirror. They are on a dimmer and they provide alot of light. I have a small chandelier in the room over the bathtub which helps balance the light in the room.