1º - GUIminer doesn't show Accepted/Stales, it just stays on 0 though when i log on the pool there are some accepted!
2º (Big One) - I can't get the 2º core to mine, after 2-3 secs after starting I just get "2011-08-25 00:47:07: Listener for "ATI6990 Core 2": pool.bitp.it:8334 25/08/2011 00:47:07, Verification failed, check hardware!"
The core seems to be working on gaming and I doubt it is an hardware problem. On some thread i heard that disabling the crossfire would solve it but the Crossfire option doesn't appear on CCC, probably because Crossfire is usual for different cards.
Which version are you using? Two versions of guiminer was released yesterday. the second release had a few fixes and I believe I read something about 0 shares being fixed.
Are you overclocked. Usually when I had a Verification failed, check hardware I had either to high of an overclock or it was to hot in the room where my mining machine is in or to high of an overclock and to hot.