ArticleID: 10060 Title: Using Multiple Monitors
The "supported" way to use X-Plane with multiple monitors is to use multiple computers on a local area network (LAN).
Through the x-plane internet options screen, you can indicate which computer is the "master", and which are the "slaves" used to display extra views.
Each machine needs to be running it's own copy of X-Plane (same version, same scenery, etc. - best to use file sharing to do this), and you must setup each computer to do this through the x-plane internet options screen.
With that said, Mac users can use a single computer and multiple monitors by setting the monitors up appropriately (displays in OS X)...
Then you would set x-plane's resolution to your combined monitor resolution (or a portion thereof). Use at your own risk!
Update 03.08.01
Several people have reported success with using 2-headed video cards and Windows ....