Nah it works with SLI hack.
Basically SLI hack allows you to SLI any two Nvidia GPUs. However, using 'odd' GPUs (per se) usually causes it to crash and become unstable.
Success with the 590 and a 580 however is quite high given the GPUs are pretty much the same.
http://www.techpowerup.com/forums/showthread.php?t=158907
It's been renamed to DifferentSLI now..