Hi Hugo,
you can indeed both outputs on you AF, using the "Speaker B from phone 2" feature . Here is how you can do this :
Open the AudioFuse Control Center
Click on "SPEAKER B FROM PHONE 2" icon located in the upper left part of AudioFuse Control Center, pictured with an arrow on the attached screenshot (no need, in your case, to click on re-amping or ground lift)
This will allow you to use the main output knob of the AudioFuse to control speaker A output volume, and you will be able to control Speaker B output volume, using Phone 2 output knob.
Hope this helps,
feel free to come back to me if needed.