Not sure if this helps, but tell me if it does:
"You need at least one card that is capable of supporting dual monitors because the resources normally taken by a single video card need to be shared across two; one card needs to share its resources with the other card. This card needs to be the master card, the card that initializes first during the boot-up process.
How do you determine which card is the master? If you're using two PCI video cards, the one nearest the CPU will boot first. If you're using one AGP and one PCI, the PCI will always boot first. In your BIOS settings you might be able to manually determine whether an AGP or PCI video card is the master. You'll have to poke around in your BIOS to find out."