thanks, I got it working - probably it was not needed to reinstall ati.12.4 driver and try to patch it for kernel 3.6.11.
I think finally the problem was misconfiguration in xorg.conf
Now I am using MATE-DESKTOP and both cards are crunching 935 Mhash/s together.
Unfortunately probably something went wrong - seems like overheating HDD controller causes to remount disks as read-only
Is this OK ?
test@localhost adl3]$ ./atitweak -s
0. AMD Radeon HD 7900 Series (:0.0)
engine clock 1150MHz, memory clock 1000MHz, core voltage 1.25VDC, performance level 3, utilization 99%
fan speed 100% (4251 RPM) (user-defined)
temperature 77 C
1. AMD Radeon HD 7800 Series (:0.1)
engine clock 1120MHz, memory clock 970MHz, core voltage 1.21VDC, performance level 2, utilization 98%
fan speed 48% (2122 RPM) (default)
temperature 73 C