Hi,
I'm Rodrigo and I'll help you.
If your system does not have the stereo mix option, you will need to use third-party software to use this setup.
The best option is to use Voicemeeter, which will give independent controls for both audio outputs, with independent volumes. It is free.
Download it here:
https://download.vb-audio.com/Download_CABLE/Vo...
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After downloading and installing it, you must restart your computer. After restarting the computer, open the Voicemeeter application, select A1 and select the one output device, and A2 and select the other output device.
On the taskbar, select the volume icon > Up arrow > Select Voicemeeter input. This will allow Voicemeeter to control your audio outputs.