I'm afraid that solution won't work, because it's for a single instance of X, which has two screens. We're talking about a solution which has to run a separate instance of X for every monitor, because it has to assign a different keyboard and mouse to each screen. Even if we got the multi-screen in one instance working, how could two (or more) users work simultaneously? They would fight each other over gaining focus for their application.
What I want to know is if there is a way to explicitly assign a monitor to one of the two ports (using two different XF86Config-4 files) without messing up the other X session's assignment?