Can I first just confirm that your setup is what I think it is - you've got a dual headed graphics card, and onboard graphics on your motherboard, and want to use all 3 at the same time?
Your BIOS settings can be found by hitting a key on boot up (there will usually be a prompt telling you what this key is, it's often F1, F2 or delete.) Look for a section such as "onboard peripherals" or similar, and see if you can find an option to set your onboard graphics to always enabled. As I said earlier though, unfortunately this option doesn't always exist - if anyone does know a workaround then I'd love to hear it!