Radeon HD 6990 - only one core (out of two) working?

Question

Video cards

Gentlemen, tell me, there are no more ideas left. I bought an HD 6990, put it on an MSI MS-7236 motherboard, only one of the two cores works (this can be seen in GPU-Z and in cgminer -n). Tried Win 7 (32 and 64 bit), a bunch of driver combinations (Catalyst 11 to 13). CrossfireX is always off (and should be on, as I understand it), only one GPU instead of two.
I decided that the second core was just dead, I changed the card to another one (also HD 6990) - exactly the same.
I changed the power supply (was a cheap 800 W, now Inwin 750 W). Only one core shows.
I tried to start the video card on the Acer M1640 board, but it does not turn on at all (although the PCI-E slot works with another video card).
What the hell is this? What could be the problem?

Answer 1

90% that software glitches, we still need to test something

Answer 2

Are you sure it's not the manufacturer's intention? the second core may not turn on immediately (well, or yes - the software incorrectly collects information). Try to run some 3dmark and compare the performance with the "reference". If there is not a big difference, then it seems that there is no problem.