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.

I will have a 70 inch vanity with a full wall mirror and 10 foot ceilings. Will a sconce that is 30” high X 8” wide X 4” deep on each side of sink be too much? Thank you for any help.

clear glass with Edison style light bulbs will give you either not enough light or an orange colored light. This is why I tend toward a frosted glass, so you can put the light bulb you like in it. IDK what stage you are at but I would tape out the mirror size you like and see where that would land sconces. What about doing a metal-framed mirror in a copper or ORB finish? I lean toward circular for some reason,

You don't have room for round mirrors, unless you want really small ones! A 60" double vanity is not big enough to fit everything you want. If you do rectangular mirrors that are as wide as the sinks, then you can do a sconce in the middle.