I have a similar problem with dual HIS 280x (i have a total of 4 actually). On two different boards with two different power supplies ( a Thermaltake 750 and a Corsaire 1000). A single card runs at 730 and cool, dual cards drop to 600/500 or 730/630 with some extensive cgminer tweaking and very hot. I don't know if it's a PSU issue, doesn't look like it in my case.
I'd of thought that 750w would be sufficient. My cards are slotted straight into the board, so only have an inch space - could be heat throttling? The 280x on top sits around 82' as it is sucking hot air straight in from the 7950 below.
Need powered risers here asap! Bought them last week and they still haven't turned up!!