How to attach a divider between windows and mini blinds

Here's a great tutorial- https://www.diynetwork.com/how-to/make-and-decorate/decorating/how-to-install-window-blinds

Use double sided tape to stick a piece of UPVC down the middle.....

Are you wanting to put a separation down the middle between the two windows or just across the top to designate that they are two separate windows? If it is the second, I built cornice boards out of foam core, hung them on small right angles from the hardware and stuck a round headed straight pin thru the foam core and into the hole of the right angle to secure them. Light weight, easy and inexpensive.

https://www.hometalk.com/diy/windows-doors/windows/a-high-end-look-for-less-foam-board-cornice-window-treatment-3704233

You can solve the problem with a single blind that fits the whole window.

So that you don't permanently disturb the plastic dividers between the windows now, you could use command strips to affix a piece of wood between the windows to help create the division. Should do the job.

There is sunlight coming through the space between the two blinds, even though the space is in front of the wall, because of where the headers to the blinds are mounted. They are mounted on the window frame, which projects the blind forward from the glass part of the window.

Option One:

If you took the blinds off and remounted the headers on the wall above the window trim, the blind would fall forward at a closer depth to the glass. As the glass is back from the wall space in between the two windows, this should eliminate any sunlight coming through.

Option Two:

Remove these blinds. Purchase new narrower width blinds and mount them back inside the window frame. As long as the blinds were the same width as the inside window width, there should no longer be any sunlight peaking through.