Windows 98 is able to run both cards at the same time (even for extended desktop setups). The only issue is with OpenGL, which works only on the card that was used for booting. Switching between cards will require just going into display properties, enable the screen connected to the secondary card and disable the one connected to the primary card (no restart required) + connect the monitor cable to the active card (if you don't have a monitor with two VGA inputs).
I would recommend you to buy a Voodoo2 instead. It's a cleaner setup.
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Regarding the difference between 8MB and 12MB versions of V2, go with 12MB one if you can. It can store twice as much texture data because it has 4MB connected to each of the two texture mapping units (instead of 2MB). Especially in early Direct3D, which loads each texture in both units, an 8MB version of V2 means that there is just 2MB of storage space for texture data... and that's not much.