Multi user mac?

Well who would have thought? I have become a Mac user. After seeing Windows Vista at my moms computer, I decided I had enough of MS. And I love it! No noise, very useful software included, very quick to start and stop, and my girl-friend and I are simply always logged in. Switching takes 2 clicks and 2 seconds. Then an idea struck me. I still have a spare monitor and attaching a second keyboard and mouse is not so hard either: why can we not work simultaneously? Unfortunately this is not supported. But why not? My question of the month: Why is the Mac still a single user system?