Of course, you can forget the two cards rendering together for the time being as Nvidia has problems with multi monitor-configurations. Basically, display resolutions are dependent on one another and have to match on both displays.
We had an accident with that at Nvidia'a booth as we broke the Vista 2 system. We did get it back in a minute or two. We have some experience with it, as we played with both betas now.
The only thing we did in the lab that made the multi-monitor work was to remove the second card, otherwise we would get a black screen every second reboot.
At this time Vista beta 2 simply doesn't work with Nvidia dual card configuration. We are anxious to try ATI but we don't expect it to be smooth either.
Vista is early 2007, if we lucky, so Nvidia and ATI still have some time to polish it up. But, for the time being, if you use SLI machines and wants to give Vista a try either enable single display or disable the second card in control panel. µ