I run spotify in the player widget and twitch in the main window. The reason is because I wanted to be able to toggle spotify on without blocking twitch, which I can do by pinning spotify in opera gx. However, sometimes adjusting the volume in twitch will pause spotify, and hitting play on spotify will pause the twitch stream I'm watching. Running multiple audio sources is very common, and opera gx should not force exclusivity to a single audio source.
Secondly, I'd like to request a feature, add a mixer widget which takes individual tabs and gx widgets as inputs. and allows the user to mix them easily.