Most likely there was no room for two fans and 2 heatsinks. So 2 heatsinks and one fan was better option in order to keep the card dual slot.
I buy the card I hope Artic Cooling will design new cooler for the HD 6990, something like the Accelero XTREME Plus
which will probably turn in 3 slot card.
Something like the limited edition Sapphire Toxic HD 5970 4GB
. It is 3 slot card and the cost was 1099 USD
Compared to that the Accelero XTREME Plus is only $75 plus $699 for the HD 6990