What is the distance from the edge of each wall to the edge of each mirror? That is the space that matters at this point. How wide is that? How wide is each sconce? How much space will you have between the edge of the sconce and the wall, and then between the sconce and the mirror if you center them on that space? You might shrink the width of your mirrors to gain more space to the far left and far right. You could have a centered sconce with two lights on it that would match single sconces on the other side of each mirror. But it appears the sconces you have chosen might be too large for the space to the left and right of your mirrors.

When you find the right size of sconce and mirror, why not put one or two sconces in the middle? The best lighting comes from both sides of a mirror.

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Yes to a middle sconce or two. You have a box wired in there already….

It is recommended to add a wall lamp in the middle of the two mirrors. This can supplement lighting, provide a more balanced light distribution, and help reduce shadow problems.

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