a) You have a Mac Pro with dual video cards, and each video card can drive two monitors. You use 2 desktop monitors and 2 additional monitors (such as projectors on HDTVs, which is how I use this setup).
b) You want to turn on Display Mirroring, but not the expected way, where all four displays to show the same image. I wanted Monitor #1 to mirror to HDTV #3, and Monitor #2 to mirror to Projector #4. Typically, once all the displays appear in the Displays pane of System Preferences, all mirroring does is display the output of Monitor #1 on all four devices.
After several months of talking with Apple Geniuses, consulting multiple AV companies and purchasing useless video splitter boxes, we accidentally discovered a solution to our problem.
Open System Preferences > Displays. Hold down the option key while you drag any monitor icon on top of any other monitor icon. You will see those two icons layered on top of each other to imply mirroring. Now you can selectively mirror any monitor to any other monitor.
We have successfully used this on Snow Leopard and Lion, on Mac Pros.
[kirkmc adds: I'm a single-monitor guy, so I can't test this. If anyone has four monitors and a Mac Pro, feel free to tell us, in the comments, if it works correctly.]

