Here is a method that would work for blending audio. The 10k resistors are needed to avoid blowing amp chip on the systems by backdriving the amp.
Other method is to get a professional mixer box found in a music shop that specializes in selling amps, mixers, instruments etc, that has the proper protection to avoid backdriving amps and blowing audio chips.
There are also software methods to cast the audio/video from one system to the other, such as casting through twitch from one system to the server and then connecting to that live stream cast with the system your at and have minimized, but audio playing to you at that computer, but the audio cast is single directional and eats up some bandwidth and processing power. A mixer box will not load down your hardware with trying to render and cast audio etc.
