It's not the best way. Here is a diagram of how I do it. Insert any intercom for the Cardo.
Stereo splitter from the Cardo headset out. One to your speakers, one into a dual audio inline volume regulator. Why? Because the Cardo level is way higher than the mic level. For the Osmo Action 3 the mic level also needs reducing.
In order to have your voice and the intercom audio on two different channels, you WOULD need a stereo to mono splitter cable, but why make it complicated? Just snip the correct wires inside the inline volume regulator. Job done.
The good thing about seperate channels is: you can adjust the volume levels seperately and you can cut out the intercom audio if you don't want to have it in the video.
Of course you can go the two mics route, but you asked for the best way. And when talking about "best" I mean best lossless audio quality.
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