If you set the primary card in BIOS each time you swap OS I cant see any problem.

Maybe if you left the ATI as primary the following would happen?
I would guess the ATI would show as a default VGA in Win98, so you may need to disable it in Device manager and force the Matrox as the Primary
WinXP would know about the Matrox (And Voodoo's) as if you were setting u a second monitor, but would default to the Primary ATI

I've never put this to the test though

Try using the Matrox Mystique as the primary card. In Windows XP you should install the default drivers for it and just disable the monitor on the Mystique when you also got the X1950 drivers installed.

I'm using this method on VMware ESXi when I pass an AMD PCIe graphic card to a Windows XP/7 guest as I can't disable VMware's emulated primary graphic adapter.

Thanks, it's good to know that it is at least doable. Also, great ideas!

Will try them the following days and update the thread.

I've tried the following:

Primary VGA --> PCI Matrox
Install XP
Install all drivers
Disable Matrox
Signal comes to X1950Pro AGP after the XP boots, but it works! Except for some black screens for 1-2 seconds when I exit a 3d program (be it 3dmark, game or dxdiag, it doesn't matter). I also think that it's got some hiccups during 3D, but I'll have to investigate further.

Next: Try the same thing (with the AGP card disabled) under Windows 98.

I'm a bit late, but yes, I did the same thing in my Retro PC with a Voodoo 3 in 98SE, and GeForce 2 MX400 in XP.

As with you, I set the bios default to PCI, and the 98 half is already happy. XP will take over and power the AGP card once it starts up.

I don't recall a black screen issue after exiting games though.

I'll have to investigate into this, because of the hiccups in the 3dmark run I tested.

It may be the saturated PCI bus (all 5 slots occupied), or some PSU issues? It may even be temperature throttling or some temporary confict with the disabled PCI primary VGA?
I'll install 98SE later today to see if the VoodooIIs have any hiccups there.

Well, just for the record, it was a broken GPU.
Hiccups under DX8, computer freezes after a few seconds under DX9.

So the whole effort of a combined system is abandoned. 🙁

Could also be power supply or cooling issues. That's a lot of cards in one PC.

I tried with a Tagan PSU of enough old-schoolness (30A @ 12v) and with the AGP card only installed.
Also, CLR CMOS etc etc. It's probably that 🙁