Here is what a review site says about the Radeon HD 6990 ....
Under the hood, the HD 6990 is nothing more than a pair of HD 6970 Cayman GPU cores that have been binned for low leakage ASICs and downclocked to meet the temperature and power commitments of the card's design. The HD5970 was built to fit into a 300 watt profile while the HD 6990 is designed to fit into a 375 watt profile with the ability to bump up to 450 watts via the AUSUM switch. This switch (used on all HD 69XX series cards) now gets a new function as an overclocking switch that loads a new voltage and clock speed profile. Something the HD 5970 lacked. This one change bumps the core clock speed and voltage up to meet HD 6970 clock speeds of 880MHz from the standard, as delivered HD 6990 clock speed of 830MHz. More changes that include refining the cooling design and voltage circuits promises a new take on the dual GPU, single PCB, video card.
For bitcoin mining purposes, you are better off installing two or more lower grade CPUs on separate cards. The HD 6990 simply fits too much processing power in a single package and when overclocked, runs very hot and uses lots of power compared to alternative configurations.
I chose to build three rigs each having 2 HD 5770 GPU cards. In total they draw 850 watts and deliver 1200 MH/sec. Because they are six separate cards I can run them in the non-air conditioned crawl space under my house.