I know this is a year old thread. but there is a way as i was looking for my computer to do 3 outputs at once. but before i had 2 outputs sorted, i had audio jack and hdmi. it's same concept. go to control panel>sound>recording>right click in blank space>Show disabled devices>Right click stereo mix>Enable>Stereo mix properties>listen>tick listen to device and then set device you want sound to play from. so make a default playing device and then in stereo mix choose the second output.
EDIT: This will not work if you have 5.1 digital optical connection on your system. The system can convert analog audio to digital and play them simultaneously but not the other way around. and if you convert analog to digital, it'll only do it as stereo sound. no matter the set up.
hope this helped.