Depending on motherboard and bois type, you should have an option in the bios to specify if opengl or pci is the primary display. Check that first if you haven't. If the bios doesn't give the option the TNT2 might have a swtich for it as I know an old pci vid card I have did.
I questioned the tnt as I had tried dual with one once before and got no where. Each card running maya on it's own is obviously a good sign, but I am not sure about the rules on mixing. They might have to be the same type of nvidia chip. I've looked for info on the past and there wasn't much about it. Might wanna e-mail the card manufac. or nvidia about it.
As for trying win2k, if you have the time might be worth trying, though I can't say it would help. I remember reading something about win2k having problems with dual opengl support and if that's the case I'd assume/hope things where improved in XP.
If you haven't, try many different drivers being sure to unistall them before trying another. Sometimes old ones actually work better then the latest when it comes to maya.
Sorry I can't be more help. This topic seems to have little factual info out on the net, so I can only go by my own experience. And as I said, I've yet to see flawless stable results from any drivers under 2k with similar cards. GF2MX AGP and MX400 PCI. Have had it work, but only to gives errors after a month or on some scenes that aren't any different from ones that work.
~Garret
