I've also found an easy way to let X use 2 screens, be able to maximize to just one of the monitors, and still drag windows around both windows.
Just follow these steps:
1. Open nvidia-settings as root
2. Configure the second monitor as 'Seperate X screen'
3. Click the 'Save to X Configuration File'
4. Configure the second monitor as 'TwinView'
5. Click the 'Save to X Configuration File' and 'Apply'
You might have to reboot, but the settings in the xorg.conf file are now ok. This is because the NVidia settings tool creates an extra entry for the second monitor for step 2, and does not remove it for step 4...
Not really needed for this thread anymore, but it might be helpful for someone ;-)