Add a tray or plant between the sinks.

There’s a wonderful golden ratio in nature dubbed the golden ratio. 1:1.618 Google it and how it is used in architecture and see how you can apply it to your situation.

I would put a small green topiary plant between the mirrors. It would fill the dead space and add some color. I really like your pivot mirrors. You also don’t have any toiletry items on your sink area which makes the space look quite sterile. Get some matching colorful soap dish, lotion bottle and toothbrush holder and that would also add interest.

I really think it's fine how it is. You could add a tallish plant (orchid?) like this:

If that doesn't work for you, I might look for something like this to hang on the wall:

Or perhaps some small pictures you could stack (3 of the same size).

It looks great. No art should be put in there. I’d recommend a beautiful orchid right in the middle.

You are right to notice something is out of balance. The sinks are far apart and there is a large space in the middle. To make matters worse the backsplash on the sides is higher and the back wall has only the vanity backsplash. But not to worry you can still bring balance...

Short of switching tge 2 mirrors for a large wall to wall mirror, a few options would be to add either a shelf, a plant or a towel rack inbetween mirrors or decorate the back wall with a wallpaper or a contrast paint. It really depends on what else is there in the bathroom and how much more effort you are willing to put in.

Here are some ideas

One large mirror all the way

Or just below the lights

Towel rack inbetween

Contrast paint and tall object inbetween

Or wallpaper

A few suggestions:

Wall to wall mirror would look right in this situation.