I have connected the PC via optical cable to the main inputs on the subwoofer and also connected an old smartphone to the headphone input on the right speaker. When both devices deliver a signal, both can be heard at the same time.
I also connected the PC via 3.5 mm to the inputs on the subwoofer. In my case, however, the optical and 3.5 mm outputs appear as separate devices in Windows, so that I cannot use both at the same time due to the software. In this case, I have to switch between them in Windows.

If I remember correctly, the unit has two audio inputs, one with a 3mm audio jack and a second optical audio input. I bought the unit because I wanted an optical audio input because I had HF radiation. However, the first audio input was HF-fixed, so I never needed the optical input.....