I wrote an Electron app (started as just a local web server) to basically do this. The UI is kind of like iPhotos or any other photo viewer except you can split view as many times as you want and views something different in each pane.
Here's a short clip of 8 videos playing
https://www.youtube.com/watch?v=nK52DY6sq9U
or a better video starting from the basic UI, starting videos, zooming them to "cover" mode, and splitting and starting more
https://www.youtube.com/watch?v=gH9h3XGkFmc
Here's the app
https://mopho-v.org
You can also open multiple windows so if you have multiple monitors you can easily run one more videos per monitor until your computer can't keep up.
note: It's pretty alpha, several bugs, but I've used it several times a week, living with it's bugs, for a couple of years. Not sure I can justify fixing it up as I'm the only person using it AFAICT but maybe it will give others inspiration. The code I'm sure is a mess. It was my first React app (a reason to give React a try).
It also works pretty good in VR as in on Oculus Rift I open my desktop and manipulate videos and splits with the touch controllers as a mouse. No particular VR support, just saying that most of the features have clickable buttons so it's easy to use in VR.
A big limit is being Electron it can only play what Chromium plays (mp4, mkv, vp8, vp9). I've been wanting to look info FFMpeg in wasm to add support for other formats, saw the post yesterday someone had done something. I figured it might be too slow and that figuring out how to sync audio might be painful but it's still on my magical "someday I might" to do list. Since Chromium already uses ffmpeg it might be easier to just patch Electron to use ffmpeg with all features enabled instead of just the few that ship with Chrome.