There’s one audio engine and it seems to happily take commands the available windows. For example I have two windows, one which is activated
Window #1: 
And one which isn’t, but happens to have the metronome turned on because by default that’s what you get a with a new blank project
Windows #2: 
Problem is when I hit play in #1 I get an unwanted metronome, apparently coming from #2. But because 2 is inactive I’m unable to disable the metronome, all those controls are sensible deactivated.
The mixin means that inactive windows can still play using whatever is active which can be handy, but this is problematic, too many years in the practice room means I can’t stand the sound of a metronome 
Additional problem, when #2 is closed the metronome flag is still set on the engine, even though #1 still has no metronome set. So it needs to be toggled to sync it all up again.