I was able to get a solid overclock on my 470 gtx when it was a single card w/o even upping the voltage (~770mhz).
When i doubled the card I definitely got improved results. I then tried running furmark on a moderate overclock for both cards (700mhz) but I find that the setup is now giving a black screen of death that requires a restart OR gives this message "The NVIDIA OpenGL driver lost connection with the display driver due to exceeding the Windows Time-Out limit and is unable to continue.The application must close."... and then requires a hard restart.
I upped the voltages to alleviate this but no luck. Temps aren't an issue either - I have a zalman on one and created "the mod" on the other with a liquid cooler. The max temps are 80 and 65 respectively.