I can see nothing under Display Properties that would help, even under Settings/Advanced. All I see there are tabs for General, Adapter, Monitor, Troubleshoot and Color Mangement. Under each of these tabs I see nothing which will allow a second monitor.
I have a Dell Latitude 510 with an ATI Radeon Mobility graphics display.
I've just updated the driver and got something a little different. Under Advanced i have Displays but only the options "Default Single Display" or "Switch Amogst Dsiplays". Nothing about using both at once as a single monitor.