As I understand it, it was possible to do this under Windows XP, but with Vista and Windows 7, it no longer is (natively).
What I ended up doing is purchasing this 4X1 HDMI 1.3b Certified Switcher w/ Toslink & Digital Coaxial Port. This allowed me to carry HDMI audio and video to my HDTV as well as TOSLink Optical audio to my surround sound system simultaneously. I did run into some issues when I installed my Ceton InfiniTV4 card. I believe they were HDCP compliance issues.
Not sure if you need them to play audio simultaneously or perhaps you just need an easier way to toggle between the two.