This is great, thanks Mochu. I used a version of this for UT99 and ut2k4, unreal gold as well and it works. Killing Floor works as well (along with the goldberg steam emulator), but still dabbling about the xinput controller (xbox one) axis freezing when shooting.
There is a team of us on discord called splitscreendreams that are using and developing an app called nucleuscoop, and writing scripts for it that does what you created for ut99. It uses goldberg emulator as well for steam games, and xinputplus for older games. Not only nucleuscoop but also Universal Split Screen which is similar and allows multiple M+KB per window as well. You are welcome to join, I can send an invite.
Here are a list of games that run out out of the box with nucleuscoop with minimal setting up (UT3 as well) along with their scripts:
Noticing that Alien Colonial Marines isnt there and should be. We have a lot more in the discord group as well that work (as well as references to other splitscreen games), but have not been developed into end user scripts and require more steps.
I got UT99 working.
I would love to hear how get split screen on UT2k4