I plugged my 12.04 laptop into a projector the other day for the first time, and dual monitors worked just fine, without doing anything.

Try fullscreen Sdl or rotating one monitor's picture.

What are you trying to say?

I use AMD on a desktop and a laptop and dual monitor works perfectly in Ubuntu 12.04 with both in various setups.

YMMV I guess. I just hooked a new monitor up to my laptop running 12.04 last night. I had to manually install the ATI drivers, then change the settings in Catalyst. I was required to reboot after each change. Then, I had to change the display settings in system settings. It took about 2 hours to get right and was a huge pain.

I have a dual monitor set up at work on 12.04 and it works flawlessly & smoothly. It is absolutely hit-or-miss. There are driver issues on Ubuntu for dual monitors.