By default a motherboard will disable any onboard video when it detects a graphics card. Some motherboards have an option in the BIOS to change this behaviour. Have a hunt around in the BIOS and see what you can find. It's often called "VGA PEG" or something like that, under Advanced Settings, or Onboard devices.
Most motherboards that I've seen have the option to force the Onboard graphics on (i.e., disable the graphics card) OR disable the onboard graphics (the default). Some have the option to turn both on at the same time... that's the setting you're hoping to find. If your BIOS doesn't have that, then you're in trouble I'm afraid.
Having said that, even a cheap graphics cards released some time in the last 5 years should support at least two displays. Can you not just run two displays off your graphics card? What's specific card you have?