There is however a very easy trick to solve this. I have tested it with an Asus Eee PC 701 4G and with my Ati Radeon X600 desktop system. I am now enjoying running dual screen with all the 3d desktops enabled.
The problem is that the default screen size configuration is too small, which does not allow you to place too screens next to each other. On the other hand Compiz can not handle texture sizes bigger than 2048x2048, which means for example two monitors of 1024x768. If you want to use bigger resolution, it is still possible, but you will need to turn off compiz and will loose the 3d effects.
So to solve it, fire up a terminal:
$ sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
$ sudo gedit /etc/X11/xorg.conf
Then find the "Screen" section and change it like this:
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
Virtual 2048 2048
To undo, you can just type:
$ sudo cp /etc/X11/xorg.conf.backup /etc/X11/xorg.conf
After doing this, you will be able to use gnome-display-properties with ease and run dual screen setups like this: