I had a bit of trouble getting Twin View to work on my new machine (Blizzard) to work.
The problem was that I could get Twin View to work when I used two of the Samsung 931BW LCD screens (my flat mate also purchased one) but it wouldn’t work with my Fujitsu Siemens screen and my Samsung. Turns out after much head scratching that there is a problem with the Fujitsu screen in that it doesn’t send any EDID data when its plugged up via DVI. Even if I specify the HorizSync and VertSync it still did not work correctly. After hundreds of restarts of X and few restarts of the computer I still couldn’t get it to work.
Then I tried with a DVI -> VGA converter and connected the fujitsu LCD with a VGA cable and things sprang to life. It appears that the LCD only sends EDID data on the Analogue connection. (I’m guessing)
If you do plan on fiddling with TwinView I highly recommend that you try out nvidia-settings it makes making changes a million times easier and makes things like positioning the screens easier too. I’ve put up a copy of my xorg.conf file in case it’ll help out someone else stuck getting Twin View to work.