ATI can get confusing. You might have installed ATI's CCC, "Catalyst Control Center", along with the display drivers. I suggest you download the latest combo-CCC and drivers for your video card. ATI comes out with new drivers pretty fast. When using dual monitors CCC is where you want to setup your monitors. After you set up the catalyst control center you'll find it much easyer to assign what monitor displays what. One other suggestion. ATI will want you to uninstall any other software for your ATI card even though all you have is ATI software.
Driver site: http://ati.amd.com/support/driver.html
ATI's uninstaller utility: http://support.ati.com/ics/support/default.asp?deptID=894&task=knowledge&questionID=1447