I personally wouldn't put too much faith in Arctic Cooling's ability to make a capable dual-gpu heatsink for the 6990. I'm stating this based off of their past attempts (accelero xtreme 4870x2 and 5970) and their next to zero support for them afterwards. Just do a quick google search for either of those two coolers and you'll see what i'm talking about.

You're best bet would be water.