Just got back from a project with a tricky vanity lighting situation. The architect is dead set on using only a row of small recessed lights over the sink.
While I can see his point from an aesthetic point of view I still cringe, since it is a horrible decision from a lighting design standpoint. It just doesn't work! Who wants to look in the mirror and see all wrinkles highlighted?
Ideally you want your face illuminated evenly from both sides. You can achieve this with two sconces mounted on either side of the mirror, with the center of the diffuser positioned at 5'6" from the floor. Choose longer fixtures if you are sharing the bathroom with someone, who is considerably shorter or taller than you.
Because of limited wall space I have pendants in my bathroom , which works just as well. Line the fixtures up with the sink so that they are approximately 36" apart (18" on either side of the center of the sink.) Depending on the size of your mirror and the lay-out of the sinks you might want to use three or four sconces for two sinks. Supplement the sconces with ceiling lights, but try not to place them right overhead.