Hmm...
The way I see it :
Both cards (Radeon/Voodoo5) will install without Problems, and there are no conflicts in Driver .dll's or the name convention.
Opengl32.dll is a System .dll, which no Driver will overwrite.
The 3dfx Driver will install the 3dfx* and glide2x/3x Core Files, and will register as a 3D Adapter to DirectX, as well as make the Registry entry to its 3dfxogl.dll, qualifiying it as a OpenGL capable device.
The Radeon will install its ATI* .dll's, and will take the same steps to DirectX and
add an additional OpenGL Registry entry towards its own OpenGL Driver .dll .
None will overwrite the other in any place, since the Driver .dll names are different.
One card will be the Primary Adapter, and the other simply the Secondary. That's how DirectX will recognize and operate them. OpenGL can be controlled by all Games, that allow the Adapter No. to be modified within their Settings (most that I know of, work fine that way).
The only Problem (not yet mentioned) might actually come from the All-in-Wonder itself :
In case it has Multi-Monitor Support, it
might very well register as 2 Display Devices for Windows overlay surfaces all by itself already (my R9700pro does, for example).
With the Voodoo5 being the third player in such a case, I have no clue how this will work, unless one disables the present, 2nd Adapter on the Radeon in the Device Manager (if present and Problems occur).
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Once that is set, however, I see no Problems that should arise from it (unless there's something in the Drivers, that inherently conflicts), and one needs a Monitor switch of course. With a nifty Auto-Switch that exist on the Market, that should be painless, though.
The one thing to take care of, is that the Card scheduled to run OpenGL Applications by default should be installed first, as (
I believe) the first OpenGL Registry entry defines the AdapterNo. 0 (default primary), and the 2nd entry AdapterNo. 1 (secondary).
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I tested a Voodoo2 running besides a R9700pro that way, Direct3D allowed for the Adapter to be selected, as well as OpenGL (in my example, from within the UT.ini of UnrealTournament). Glide of course was automatically handled by the Voodoo2.
Basically, I then had 2 Direct3D Devices, 2 OpenGL Devices and 1 Glide Devices in the System, and all selections worked as expected, no conflicts
DirectX nowaday's is smart enough to allow for multiple Devices (the times of conflicts, forcing one to disable DirectX support for one card, are dated back to Matrox G100/Mystique & Voodoo1 times). OpenGL had this intelligence built-in since a long time.
In case of Problems, one can still disable DirectX Functions on either of the Cards from within the DXdiag Applet.
---- edit ---- Thinking about it, I
believe I've heard something, that
Direct3D Hardware Acceleration on a true Secondary Display adapter might be impossible. Having never run 2 full Video Cards besides each other, I can't confirm this, but at least that's what I read about it... Someone would need to test it.