said by Apple :Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors.
Cool. It did surprise me that Apple would say that MBA with the same HD4000 as the rMBP would drive "an external" instead of the 2 it claims for that rMBP. -- In a nation of spoiled children, Santa Claus always wins.
It's probably like some of the RAM capacity specs...it can do it but it isn't "supported". Maybe Apple's T-Bolts displays work while other displays end up not, or flickering on and off (like my Dell did with the T-Bold to dual link adapter). -- In a nation of spoiled children, Santa Claus always wins.