Did you install Hydra-vision? That's what is for ...
You need to activiate it in the "Display properties", "Setting", "Advance" button ... if I remember correctly, from there you get a whole set of ATI options (Sorry, I am not at my computer). In there you need to turn on "display in multiple screens" (or something like that).
Basically, you have to turn on Hydra-vision ... Heh, stupid ATI didn't have a manual for these things, those idiots!