Selected Audio Output Change

actualy this is a general JACK question and quite frankly would be one completely moot if you were using pipewire(-jack) instead of genuine jackd (are you?)

now, you know that in jackd you can only use or select one audio device interface and only one, don't you? if not for some reason you have forgot than write that down as a capital-R rule, again please ;)

also, when you have jackd (via qjackctl) up and running on the selected audio device interface, it takes full and exclusive control of it, so that it won't be available nor working to any other application, desktop, control system or else... but only to jack-aware client applications that is.

there are several ways to make other audio devices to collaborate in this draconian jackd rule, some are more or less complicated, overly technical, far from flexible and quite uneasy to grasp for the casual user (IMHO).

anyway, all of those ways are old and dated fashion and sorry to tell, I'm failing the time to enumerate all the ways the problem/question may be workaround... let alone to detail explaining any of them (none of them are specific to qjackctl alone, mind you).

okay, the best I can do is to suggest you take the plunge to PipeWire and be happy ever after :)
cheers