I've been trying for a while to figure out how to get dual monitors to work on Debian Linux. Finally, they're working, and it was pretty easy.
The missing link was here:
I couldn't find this because I was searching for "dual monitors in Debian Linux", but the standard documentation was for Gnome.
This is a laptop with an external monitor. Here are the steps:
1. Turn off the computer
2. Connect the external monitor to the back of the lap top.
3. Turn on the external monitor.
4. Turn on the laptop. (It booted up and the two screens were identical, aka "cloned" or "mirrored".)
5. Log in.
6. Go to System -> Preferences -> Monitors (which is part of Gnome)
7. This showed a box titled "Monitor Preferences" (as described in the URL above). A box representing the external monitor (in my case, it said "SONY 18") showed inside a larger box. It was a little confusing because the box representing the external monitor was I guess covering the box representing the laptop.
8. Drag the box representing the external monitor so you can see the laptop box "below" it. Continue dragging it until both boxes are side by side in the same orientation as the physical screens. For example, in my case the "SONY 18" external monitor is physically to the left of my laptop, so I arranged the boxes in the dialog to reflect this: the "SONY 18" box on the left, and the "Laptop" box next to it on the right.
9. Click "Apply".
Searching the web in Google shows that lots of things can go wrong setting up dual monitors. But these steps were literally all I had to do. If this doesn't work, the only thing I can think of is that I recently updated my system, so you might try "sudo aptitude update". I don't why this worked - maybe it worked in the previous version and I just hadn't tried it. My current version is Debian 6.0.4 (Squeeze).
That's it! No mucking about with randr, grandr, xrandr, or xinerama or anything else. Hope this helps someone.