I am assuming this runs two separate instances of the game in some isolation/VM-like separation. Pretty cool project!
Here is an example of the program modifying Left 4 Dead 2 so that it can run a separate instance. https://youtu.be/1RHWgHh2Q6M?t=120
Others require a Steam API emulator: https://universalsplitscreen.github.io/docs/goldberg/
> Some games use Steam to prevent you from launching multiple instances, and some games use Steam for matchmaking, preventing you playing with one copy.
> The Goldberg emulator replaces the Steam API dll, tricking games into letting you launch multiple instances and join the same server.
There are also a few warnings on the site that the hooks used by universal split screen will likely trip anticheat software.
Yikes, they should make that warning more prominent. Like a big yellow banner across the top of the landing page.
If you trip VAC for example, you're banned from online play for all of your VAC-enabled Steam games, IIRC. I imagine programs like this that do all sorts of weird Windows API plumbing/injection are pretty likely to look anomalous to even the most basic anti-cheat.
Vac is also only triggered if you try to sign into VAC-secured servers and VAC detects a cheat. In the unlikely event that something like this is ever classified as a cheat (unlikely since valve manually detects and adds cheats to the blacklist), simply running a local server would not trigger a VAC ban.