I've successfully run a dual monitor setup with an Nvidia AGP card and an ATI PCI card, with both sets of drivers installed--so to say "it won't work" might be a little short sided.
I agree, I don't think "it won't work" is the right answer. I don't really think there is or will be an easy out-of-the-box solution to this problem. I'm not really sure that Nvidia would create a driver to make one of their cards look solely like a physics card. At the same time, if you really understand how the drivers work, you may be able to gerry-rig something. Just make sure that the display drivers default to ATI and have the Nvidia physics acceleration drivers... or something