After looking at some of the setup guides for some of the games it doesn't look like it is using any fancy isolation tricks. So far it looks like it works with games that either allow the user to run multiple instances or the program somehow modifies the game to allow it to run a separate instance. Although I don't know what is being changed that allows the games to run multiple instances.
> 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.
> 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.
No, you're only banned from playing the game that VAC detected a cheat in, unless it's a half-life or source based game where it bans you from related games using the same engine. http://www.vacbanned.com/static/information
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.
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