man you can push those things like crazy. a single 7970 with a heafty overclock can beat a 6990/590.
the highest ive seen is 1250/1900...
ramp them up a step, stress test with furmark/a intensive game, if it passes raise the clock speed. rinse and repeat, until you get artifacts, then raise the voltage, stress test, if it passes raise it another step, stress test, and pump up the voltage... it could take up to 4-5 hours to find the perfect balance between stability, performance and heat.
but remember, you are working with 2 cards, its almost 100% guaranteed that you wont get as high of clocks with 2 cards as you could with one card... just the way it is.
or you could do what i do and go to 1.5v and put the clock speed all the way up and hope it sticks...