When a cable is plugged in, check the open windows and the system tray, a 
dialog might pop up to allow you to decide the function of the plug you just 
plugged in to the jack.
So why not the splitter?  Shouldn't cost you more than $5 and will save lots 
of aggravation?
The agenda might be to learn more about windows 7, rather than to get audio 
out to two sources.  That's a fine agenda.
After all, we are hams.  If we wanted to talk to somebody in Poland, their 
are lots of easier ways than radio.  The point is to push the limits of the 
conditions, your station, and your skill.