NVIDIa on Ubuntu 12.04 took about 10 minutes to work out twinview. You should put your method on Ubuntuforums or askubuntu. There are always threads running about AMD cards on those forums.
While external monitors with twinview work pretty well, it is a huge pain to configure. You have to open up the nvidia configuration application and set it up, not autodetection or panel applet.
Yes, I should have said that, nvidia settings just bypasses the display applet. Didn't take ages though although not friendly.
Ten bucks says you are using proprietary drivers...
Yup, can't get Unity (Ubuntu) or GS (Debian) otherwise. Gnome Shell works with nouveau drivers on Fedora 17 beta although it is clunky.
Have you tried Debian unstable?
Not yet, I was using Testing with GS 3.2. Might give Sid a go over the four day bank holiday weekend we have in the UK for Her Majesty.