I connected 2 monitors to my Radeon X 600 256 MB video card. I am attempting to run both monitors in clone (displaying same) but "display settings" only enable setting the second monitor to extend the view of the first.
OS- Windows XP Media Center Edition

Any advice?

As usual install the latest drivers. Have you tried these instructions from ATI/AMD Catalyst Control Center ?

Enable Clone Mode

If the secondary device is disabled:

From the Tree View pane, click Displays Manager.
Click and drag the display device icon in Attached displays currently disabled to the empty box to the right of the Main box.
Click Clone Main with [display device] in the pop-up menu.
If the secondary display is enabled:

Right click the display device icon in Desktop 2 when in Extended mode, Rightmost when in Stretch Horizontal mode, or Lower in Stretch Vertical mode.
Click Clone Main with [display device].
Click Yes to the Displays Manager Notification dialog.
Note: Display device can be CRT, DFP, TV, or HDTV.