As far as I am aware, what you want is not possible.
Reason for that is that the usb is seen as completely different sound card compared to normal 3.5mm jacks.
If you have the soundcard driver and software installed (most likely realtek, 95% of motherboards use realtek) you can reassign jacks to work differently than they are meant to.
As example, have stereo speakers plugged where they are now and plug in headphones and it would as what to use it (the jack you plug them to, you could use almost any jack for any specific purpose.) Selecting speakers for both will usually let you head same things from both at once.