My setup has a 17" monitor that the pc normally uses and a 21" monitor my mac is normally on. I wanted to allow my pc to switch to the 21" monitor for Director and PowerPoint work. Sharing two monitors with two computers requires two switchboxes. At setting AA, it's pc/17 and mac/21, at BB, it's pc/21 and mac/off. I can't get the mac to show up on the 17" monitor because it requires an adapter that I don't want to use for the 21".
In short, you need a switchbox with monitor ports to switch the monitor between the pc and mac. I picked mine up at the local computer parts fair because places like CompUsa and Best Buy didn't carry a monitor switchbox. You can probably find one online for less than $20.