Exactly what object are you using and what language version (USA, INT, AUS, etc.)?
Someone using the same language version will need to answer this.
In the AUS library there is a "Square Bay Window 24" (it will be the same in 23), but it looks like the sashes are divided equally and can not be adjusted (for us).
In your image they don't look equal, which makes me suspect you are using a different language version to me (and Ling), and there could possibly be a setting we don't know about.
If you make the window wider - are they still of the same proportion widths?
I believe the AUS library is the same as the INT library, but it is different to the USA one.
If it is not adjustable, then you may have to model it with separate front and side windows.
Hopefully then with a standard window in the front, you can adjust the widths of the sashes.
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