Is the GeForce GTX 590 made up of two GTX 560 or 570 cores? Putting two GTX 580 cores on a single PCB would be virtually impossible because of the enormous power requirements and heat production. The Radeon HD 6990 is made from two HD 6970 cores. The HD 6970 is almost on par with the performance of the GTX 570. There is only about a 5-10% clock-for-clock performance differential between the two cards.
I believe ASUS is making it's "Mars" edition of the GTX 590. but I don't see how they can possibly put two GTX 580 cores on a single PCB given all of the inherent problems of that task. If they do, the cores would have to be severely downclocked and "crippled," and the PCB would have to be extremely large. And I doubt that even two 8-pin power connectors from a 1,500W power supply could satisfy the power requirements of such a graphics card.