I'll make a better post about this later, but I just had to share because I think this is nuts. I "accidentally" managed to get dual-GPUs and multi-monitors working in Win98. Okay, I vaguely knew this was possible, but I thought it was for specific matching workstation GPUs with specialized drivers (i.e. Matrox). But no, apparently you can use whatever you want, and the way you set it up is (1) install the video card and the normal driver you'd always install, and (2) there is no step two.
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^^ that's a Voodoo3 driving the left half of the desktop, and an ATI Rage running the right half. Head asplode.
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I didn't even have to mess around with hardware profiles or disabling specific drivers. I just put the V3 in, installed its software, and it worked.
There are some caveats with this setup, the big one being that not every card is supported as primary video. The specific list of supported cards includes most ATI Rage models but seemingly no 3DFX ones, and I've also found some forum scuttlebutt saying it works better if the PCI card is primary. That's how I have it set up here, the system is set to init the PCI Rage first in the BIOS, then Windows starts the AGP Voodoo as it boots up.
As far as I can tell, Direct3D & OpelGL stuff always runs on the primary card, which is the Rage. D3D is pretty painless but OGL is a bit fiddly and unstable with both heads active.
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I let FInal Reality run all the way through and got exactly the same scores as when the Rage was installed on its own.
ATI CIF progs "just run" on the Rage, as you'd expect:
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...while GLIDE stuff jumps over to the Voodoo card. How cool is that?
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