I can't comment on your specific card but it worked fine for me with some GTX275 in SLI.
As you expect you don't get the speed boost and the other card sits there doing nothing but it's fine.
The only slight issue I found is that with GTX275 in SLI, the fans start at 100%. If you don't boot into windows before OSX, they stay there so if I was booting form cold, I'd have to boot into windows so the drivers kicked in and slowed the fans, then reboot to OSX.