Wow - that seems like A LOT of lighting for a 54' vanity!

I am finishing up the renovation of my primary bathroom. I have two mirrors also that will be used over the sinks (24" wide - so, just slightly wider than your mirrors). Additionally, I have three sconces - they will be mounted the same way your "lighting person" has recommended. I will not be installing any lighting over the mirrors.

There will be other lighting in the room - a recessed light over the shower + a chandelier type light over the tub (FYI- the ceiling over the tub is 11' = there is enough height in order to meet code requirements) + automatic under cabinet lighting for at night + separate light in WC.

I don't feel the need to also add sconces above my mirrors - much less two sconces above each mirror - and my vanity has been custom built in order to fit in a 83" alcove.

My first thought is that 54' seems a little bit tight to have two sinks. Regardless, I can't imgaine adding sconces over my mirrors - I don't think that there is a need for the extra light. Do you have a drawing of how the wall will look? It seems like it will look crowded (even without all of the additional sconces over the mirrors).

54" - 40" (20" x 2 for your mirrors) = 14"/3 = 4.66" of space for each sconce. You need to have some blank space between the edge of the mirrors + the wall (on the right side).

What are the dimensions of your sinks? I assume that you will want to have your mirrors centered over your sinks - I believe that code requires a 15" clearance to the wall.

I think that you need to grab a piece of graph paper in order to draw where everything will be located. I think that you will see that you don't have room for three sconces + adding two more sconces over each mirror will make the space look very crowded.

Hopefully, a pro will come by and provide some insight for you also!