Most onboard graphics will not support dual monitors, especially the use of the onboard and another video card at the same time.
Onboard video is usually designed so that when another Video Card device is detected (in agp or PCI) it will default to that instead of the onboard, but the way it does that is by disabling the onboard.
The only way of doing this i suspect is to buy another video card, although the ATI Rage is very old, you can pick similiar cards up for $10-$20 or less.. or £5-£15 here in the uk, search ebay or local adds, or just buy a ATI 7000 32mb, costs about $20/£16