If they are hella efficient at 500 watts, they'll still be hella efficient at 1,000 watts. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
But yes, the pair would be slightly more efficient due to power compression on the 1000 watt setup if nothing else. The two at half power would last a good while longer than would a single on 1kwt as well.