I wanted the windows docked together with a slider between them.
For now, I just set them up manually on a separate desktop that I can Control+RightArrow over to when needed.
Maybe, I'll write a Swift UI app with a split view container and place finder apps in them. That might be better than Split View.
I'm going to download Mosaic and see if it does this. Maybe I can use Mosaic in combination with Keyboard Maestro.