Hi
the problem is the 7850, I had that problem too.
the solution I found is the following:
download drivers (12.8 or 12.4) from amd
I used the following command to unpack the drivers
./ati-driver-installer-12-4-x86_64.run --extract
I then copied the new control file into the old place
cp /etc/ati/control /etc/ati/control-old
cp common/etc/ati/control /etc/ati/control
reboot and try again sudo aticonfig --lsa
if it is listed then try if you can get temps sudo aticonfig --adapter=all --odgt
if at this point you can get temps from BOTH cards, you are done, nothing else is required, if not, configure:
exit X (stop lightdm) then init 3
in command line sudo aticonfig --adapter=all --initial -f
sudo sync (if you are booting off usb)
sudo reboot
that should do it.
let me know if it worked for you
regards
Indkt
BTW: if it doesn't work for you, let me know, I will email you my control file that works with the 7850
if it works LTC: LaJQHWGMBcLqkBKNL8tjVYAKBtoYEBPVJA