I have done this with both an AGP nVidia and a PCI S3 video cards previously on my Slot 1 computer. Mainly to extend DOS compatibility by adding 15bpp and low resolution VESA video modes as VBEPlus and VBE15BPP (not a great idea in practice) weren't perfect. This was way prior wbc releasing a patch for UniVBE that allowed the utility to work with older nVidia cards.

The IRQ should be different for each video card, and I recommend using multiple hardware profiles in Windows.

My experience is that 98SE copes with dual graphics cards, but earlier versions may not. Also windows is going to have it's own ideas about what the primary graphics adapter is. So you might set Rage to primary in BIOS, get your POST and "Starting Windows 98" messages on it, then it may go blank and loading screen and desktop go to the geForce. This is gonna be frustrating if you don't either have a monitor switcher or dual monitors. (Or maybe a monitor that can switch inputs) In theory, you should be able to choose the DirectX output device when starting things. But I think some stuff doesn't ask first, launches on what it feels like then you have to get into the settings screen to change it.

The BIOS option does not allow switching between cards, it is to choose which display adapter is initialized first. At minimum that selects where the POST messages will be shown. Further effects depend on the VGA BIOS in question - some will only work if initialized first (typically: older PCI cards), so if you choose AGP first, you only get AGP and the PCI card is also not available in OS. Later cards tend to be more flexible.

Windows has its own primary adapter, which need not be the same as what is selected in BIOS. However the adapter needs to be initialized correctly by BIOS for Windows to be able to use it, so if a card needs to be initialized first by BIOS, you need to respect that. There's nothing stopping you choosing a different adapter as primary in Windows though.

Win98Se is actually pretty flexible when it comes to mixing and matching cards. You can extend your desktop over whatever you have. However overlay (video, games) only works on the primary display driver, so unless you have an explicitly dual-head card with ditto driver (eg. Matrox G4xx), that sort of thing will only work on the Windows primary display.

Oh yeah and boards with onboard graphics tend to be particularly weird about everything, even with it disabled, it might not recognise more than one additional adapter. Also with it using onboard as BIOS primary, then it probably won't initialise anything else 2nd or 3rd for Windows to even pick it up. Things got better with that later in the 2000s past 2007 or so, but older boards can be a real pain.