Really well execution of the idea, Our team had a same concept but didn't manage to upload on time cause Unity takes ages to build, anyway, how did you manage to make a build in window? (so that it isn't fullscreen)
Viewing post in Splitting Image jam comments
Edit > Project Setting > Player > Resolution and Presentation
You can mess around with the screen mode, screen size, aspect ratio etc. I set mine to windowed with a set resolution. This can have unintended consequences though when opening the game on screens that aren't a similar resolution to your own so I am sure there is a better way of doing it