The changes may need a restart of pulseaudio to take effect either by logging out and back in to your session or by running pulseaudio -k in a terminal. In this example it is shown for a HDMI-device but as soon as your Bluetooth device is recognized it will also be available for simultaneous output. The additionally created audio output device for simultaneous output may be selected in the 'Output' tab from pulseaudio sound preferences menu:
Then in terminal run paprefs, select 'Simultaneous Output' tab, and check 'Add virtual output for simultaneous output on all local sounds cards'.
With paprefs you have access to a virtual output device that enables simultaneous output to all attached sound cards/devices: sudo apt install paprefs
