Consider the view out and in to the bath , what sees in? What do you see out? what window covering is acceptable if you need one? Ex. If it is a ground level window looking directly into your glass shower surround and the view out is the neighbors kitchen window , that might not be desirable ... or perhaps one location looks into a lovely private garden or has good natural light exposure

so it will actually be a third floor looking out onto a field. so both views are the same and pretty! but great point!!

Consider how it will impact your light fixture choices for the vanity. For example, if you're planning on sconces, they might not fit with a window in between the sinks.

Most folks want a mirror where you're fixin' to put a window.

If you have a view to a field, then by all means put in a window in your bathroom! Natural light is so lovely, and if that were my bathroom, I'd add two windows--one between the sinks and another to the right of the vanity! Mirrors would go above each sink, not between them. If you don't have room for sconces, then use light fixtures above the mirrors to light the vanity areas.

This is my master bathroom, which also has three additional windows!

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What about a high clerestory window? Then you can still have mirrors and sconces on that wall, plus more natural light.

Are you a makeup artist? Do you want direct sunlight while applying makeup or on the side!