Certain PCI cards simply won't work in a multi-head configuration, whether from bios or driver problems seems unclear. I've had exactly the same problem with multiheaded use under windows 98 and 2K. The solution was simply to try different PCI cards until one worked. It helped to have a box full of old PCI video cards in the attic

In general, I've found that of older cards, S3 ones seem to have the highest probability of being happy alongside AGP cards. SIS ones, on the other hand...

I eventually got a PCI Geforce2 MX440 card to go along with the dual output AGP version of the same thing, and that all works really well.

pca