Overclocking
Placing a second card into a system generally means the overclock isn't as good as a single card's.
But we still managed to increase the core clock by 100MHz and memory by almost 600MHz.
The added oomph means some more gravy on top, and, if you want to compare against your own machine(s), two cards score almost 8,000 marks in the stringent 3DMark Extreme test.