I came across this Q&A session with the Media Center developers.
It's a shame that one question they didn't answer one question they could have
Sidd [Microsoft] (Expert):
Q: Is there a way to enable simultaneous audio outputs for 5.1/7.1 to a home theater setup and stereo audio for Multi Room Audio? We know the OS does not prevent it nor do the audio drivers, so it would seem that Windows Media Center has the ability to do it
A: DrFlick are you saying you have tried doing this and were unable to do it?