I have a UD9 and running a 990x CPU on it. I also have a SLI setup and pretty much all my card slots are filled.
The 8-pin power connectors from my CPU were just a tad bit too short to reach from my PSU to the MB and I only had one extension cable available when I built my system. So at first I was only using one - and my system were unstable with a few bluescreens until I got a second extension cable and connected the second 8-pin power connector. After that my system has been rock stable.
So in my case, bot the 8-pin PSU connectors were necessary for my system to run stable. if you're running a non-SLI setup with just a few PCIe slots filled then you might get away with only using one of the connectors, but then again there is no reason not to use both.