The older drivers don't work with the cayman based cards.
Did you get the catalyst drivers from the AMD website or via apt?
I am using the catalyst drivers from the AMD website. Are you using the apt version?
I don't think I know the "verification error message". I was running Ubuntu 11.04 and didn't the same problems as you.
to me it looks like the second device does not do any calculation at all. At least: The "verification error message" is triggered when a rough plausibility check of the results shows some bits to be zero which should not be zero. It looks like the driver ignores the params to the GPU and just returns default or zero results even without proper error message.
However: There are miners using more than 2 odr even 3 6990 GPUs so I assume there is a trick.