with me it is connected to the mobile and PC at the same time, it keeps the connection but then it only plays one thing, e.g. when I watch YouTube on the PC and then a call comes in which I accept, it switches to the phone

Yes you can connect 2 devices, it switches between both.

Hello, as already mentioned, the headset connects to up to 2 devices at the same time, whereby only one of them can be heard.
Information on the number of connections can be found on the manufacturer's website, either under 'Technical data' - 'Connectivity' or in the manual.

I don't know how or where you can see this, but it works. I have connected my mobile phone and PC, receive call signals from both and can switch between them.