I think it might look a bit odd but there might be some things you can do.

Trim out the mirror to match the window so it's the same size and frame. This might make it look like 2 windows.

or

If there is room on the wall and $ in the budget can you put in a 2nd window right beside it? If you don't have a great view you could suspend mirrors in front of the window.

or

Instead of 2 sinks how about one big one with 2 faucets? There are some really cool looking trough-like sinks.

Just some thoughts. Good luck!

I like lotteryticket's suggestions about using the framing to coordinate the mirror and window if adding a second window is not practical.

If you wanted to forgo the second sink you could always put a small chest or dresser under the window and you would still get a lot of additional storage and counter space.

Another idea (I have no idea about your layout) would be to place the two sinks/vanities on different walls so one isn't directly under the window.

Here are a few ideas, check Houzz for more:

[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Cambridge Architects & Designers Charles R Myer & Partners, Ltd

Here there is only the window above the second (near) sink:

[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Oakland General Contractors Mueller Nicholls Cabinets and Construction

Suspend the mirror over the window:

[Modern Bathroom[(https://www.houzz.com/photos/modern-bathroom-ideas-phbr1-bp~t_712~s_2105) by Sonoma Architects & Designers Amy A. Alper

Oooh, I like those ideas and the pictures. Thanks!!

Our master bathroom is quite small (7' x 7.5') with similar issue (vanity wall has window). We decided to go with a wall to wall 82" vanity but just have one sink (similar to the first picture except our cabinets are all enclosed).

We just figured the bathroom is so small that it would be too tight for two people to be in it at the same time. Not to mention we never seem to need the sink at the same time anyway.

Our goal was maximum storage space and this configuration allowed us one sink cabinet for typical cleaning supplies and such, one 3-drawer cabinet, and one large cabinet with two pull out shelves. Lots of storage!

Our neighbor has the same layout and she went with a sink/makeup vanity combo.

Just decide on what your needs are and what is most important (two sinks, maximum storage, makeup area, etc.) and that will help you figure out what will be best. I have seen a lot of configurations with windows. Just search Google images or Houzz.

I would attach a picture of our bathroom but it is still in progress. Instead I posted a picture of one of my favorite "sink under window" bathrooms.