ATI/NVidia multi-monitor on XP?

I'm attempting to set up multi monitors on a Compaq SR2173WM with an Asus A8M2N-LA motherboard which has NVidia 6150 integrated video. I've added an Asus EAH3450 card, which is an ATI Radeon chipset (I installed the latest ATI Catalyst driver for XP). Both are installed in Windows correctly: If I tell the BIOS to use the onboard video, then boot Windows, the NVidia 6150 works fine, and if I tell the BIOS to use the PCI-e slot video, then the EAH3450 works fine. But multi-monitor mode will not work either way. Regardless of which one is selected in BIOS, the Windows Device Manager sees only that display adapter and not the other one. Is there any way to get this to work? BTW, it works fine under Windows7RC; but I'm trying to get it to work on my XP Pro installation. TIA - JR