Have you enabled multi-monitor support in the BIOS? (Settings\Advanced)
By default it would seem to be disabled, the manual states:
"IGD Multi-Monitor [Disabled]
Enables or disables the multi-screen output from integrated graphics and external
graphics...
By default it would seem to be disabled, the manual states:
"IGD Multi-Monitor [Disabled]
Enables or disables the multi-screen output from integrated graphics and external
graphics...