I think you've found your issue, The IIC is a very old card and no dual monitor drivers were developed. Since you have one NVidia card already, I will share that I installed a PCI NVidia card and that worked nicely for a dual display without using a DUAL-output display card.
I am trying to run two monitors at the same time. I have the first card installed fine (NVIDIA GeForce4 MX 420 PCI), which supports the dual monitor feature. I installed the second card (ATI 3D Rage IIC PCI), but windows cannot install the driver for it. The device manager says "Device cannot be started (code 10)". I tried reinstalling the card, troubleshooting everything...Anybody have any suggestions as to why windows wont install the driver for this card?