I only have a little bit of experience with dual monitoring on Windows 98 (SE), so please bear with me.  My retro gaming PC has a dual boot of Windows 98 SE and XP, and is in actuallity  a Dell B1000r.  So far, dual monitoring works differently on both OSes.

1. Win98: Each card works okay and I can even create one double length display (sometimes), but I can't tell Windows which card to use as the primary display adapter, which is a bad thing because most games use the default display adapter to render with.  Worse, since the Riva128 is an AGP card, it is automatically the default display adapter while the Voodoo 3 is always secondary.  This means that whenever I attempt to run anything that requires Glide to run, it will ignore the Voodoo 3 entirely and show an error as if the card doesn't exist.  It's defeating the purpose of using that card...

2. WinXP: Unlike above, I can choose the default display adapter without problems.  It generally works okay, but I can't combine both monitors into one display (except for the desktop) and the mouse cursor is always limited to one monitor.  It's not a big deal, but I'd still like to be able to do this if possible.

I'm not to worried about the WinXP issues, mainly the Win98 ones and being able to switch primary adapters.  Would this mean I have to disconnect my monitor every time I have to use the secondary adapter?  This sucks.  Any ideas?  Thanks.

Oh yeah, my specs are below if necessary to know:
Model: Dell B1000r
OSes: Win98/XP
CPU: Intel Pentium III 1Ghz
GPU1: STB Velocity 128 (Riva 128) 4mb AGP
GPU2: Unknown 3Dfx Voodoo3 16mb PCI
RAM: 384mb
Sound: Creative Sound Blaster Live

Shogun.

With many hours of searching, I found out that I can use my BIOS settings to use the PCI card as the primary display adapter as opposed to the AGP and switch as necessary for Windows 98.  It's more work, but it works.

Btw, why a Voodoo 2?  I own one, but would a Voodoo 2 [SLI] be more stable/suitable somehow?  I noticed that even when the Voodoo card is installed properly, it glide apps don't default to that monitor unless it's either the primary adapter or unless the game is setup to use the secondary one.