The expected behaviour on this is that the software driver automatically enables or disables each controller depending on the processing requirements. I believe that this is controlled by a software called ucidLogix® Virtu (you may need to confirm with Asus*).
In this case what you should check with your motherboard manufacturer if there is a setting that will allow you to manually switch the controllers.
actually i've tried with and without the lucid virtu enabled or installed and i've reinstalled windows several times and tried different configurations. for now the only i can get directx support back to my gtx570 card is reinstalling the nvidia drivers every time i boot the machine. so i'm sure that it's a driver related issue it only remains to see which manufacter brand will come with a solution,