Im doing test with just one 6990 and nothing more and is all screwed... without activating the CrossfireX mode in Catalyst Guiminer only shows one device and is just one core because 330 mhash/s
If I active the CrossfireX mode in Catalyst I see 2 devices in Guiminer (both cores) but after few seconds mining one device shows "Verification failed, check hardware!" erros like crazy!
any help please?!
This *sounds* like a driver problem. It's windows so the amount of low-level troubleshooting one can do is limited. Clean install is an option.
If Linux is an option for you. There's plenty of experience on this board. I'm running the same 6990 using Gentoo (but I think most people here use Ubuntu). I'm even running it in parallel with a GTX480 without too much trouble.