I've seen an article on this somewhere and I'll try and find it so I can post a link.
I know that ATI's version "Crossfire" is being designed to work with different Bios versions so you can mix and match cards from different manufactures etc.I believe that the next version of NVidia's SLI will do the same.......after all it's not ideal that in reality you need to buy a matched pair of cards for it to function correctly.
One of the selling points being you could add a second card later to upgrade your graphics performance.........not that easy as you have found out.I think I'll avoid these problems by sticking with a single card solution !(er ok, I can't afford an SLI rig,LOL)
I'll post the link if I can find it.