Are you sure? i can't imagine this to be impossible, quite tricky of course but i can't help but think it's gotta be possible, even if it required two cpu's or whatever. Cause i mean, they have Multiboxing technology out there, i just don't know how extensive it gets.
There used to be a PCI card you could install for a second user but I haven't seen one of those in years. Post back when you find what you're looking for.