Two suggestions:
You could run the standa alone MG and use the controller on the midi input of the ineterface and just go out to an audio input of your cubase.
Or maybe you need to use two inputs like when one uses a vocorder like TO-VOC12 where one input is the instrument being payed and the other is the controlling envelope audio track like a voice. See how the TO-VOC12 is set up in cubase in this link –
:https://www.youtube.com/watch?v=5Cfh3C9wykI
I don’t know if it will work with the breath controller