You don't need to use ATI's application for dual monitors unless you're mirroring them. ATI sets their cards up as primary and secondary displays so anyone can easily use windows Display Manager, clicking on the disabled monitor, then checking the box "Extend my desktop to this device", clicking apply, and dragging the monitor to the position you want it in relation to the original display.
For nVidia dual displays you have to use nVidia's application, which can be found by bringing up nVidia's custom configuration window. Right click on the screen, choose the nVidia display manager tab, and it brings up a custom box with all the options required to figure it out.