Sorry GR, can't help you there. It doesn't sound likely that you'ld need 2 drivers but then I'm pretty inept at tech stuff anyway. I just scrape by mostly, with help from people like yourself.
I bet you'll get some amazing results from 2 GTXs. So far I'm dead pleased with my GTS but I haven't really given it a lengthy test in games yet. Also if 169.25 remains stable I think I'll stick with it for a while rather than trying newer drivers. I think that's what screwed my system with the other card.
Can't remember if I said anything bad about Overclockers,co.uk but I was getting frantic with the Gigabyte card. Anyway they have given me a refund and been totally honourable so I would recommend them to anyone.
Why not mail Nvidia direct about SLI drivers? Or maybe there will be something in their knowledge base/FAQs. Good luck and let us know how they perform once you get them running. John