I would like to play music (nuryl) on the bluetooth speaker (for my son), while listening to a different app (siriusxm) via headphones. Possible? Iphone 6S. Thank you for any information.
No - the system controls the output so it’s not easy for an app to remember a different output and override the system choice.
Unless Apple releases a new version of Workflow or some automation for iOS, you can’t script a change to the output device and then launch an app.