Not sure if this helps,
But I use SDK 2.1, and drivers 11.12 with 3 5970s and 1 5870 in a rig. (7 GPUs) I have not gotten any errors at all, or YET.
Make sure you don't install the actual SDK EXE installation, but extract it with WinRAR and only install the SDK MSI file. That got my hash rate up a bit.
Ah, ok, I shall do it this way instead then. Which .msi-file is it exactly? SDK_dev?
I extracted "ATIStreamSDK_dev.msi" from "ati-stream-sdk-v2.1-vista-win7-64.exe" using WinRAR, installed it, saw my hash rate improve.
If you have WinRAR just right click and choose extract to. Make sure all old drivers, SDK and other stuff have all been completely removed.
Also if you can, just reformat. Or completely remove your drivers, and install 11.12 first.
I've tried this now and my problem still remains... I also tried installing 2.1 in a different folder and it didn't help.
I think I'm gonna give up on this now since nothing I do seems to work. When Win8 comes out I'll be getting a new computer with it, then I'll probably install Win7 on the server instead and then I'll see if I can make it work on both cores. In the meantime I'll just have to settle with one core..