Set it to "Tile" and off you go.
Lots of good free dual (or triple) wallpaper out on Google.
Digital Blasphemy -
http://www.digitalblasphemy.com/ - is Ryan Bliss's site and has tons of gorgeous dual and single WP, created by himself. Well worth the small fee.
I've had dual monitors for yonks - can't imagine going back to a single now.
Looks even better in Linux
