- Edited
You are on a Mac, right?
I don't know why but it seems that universal apps like Stage Traxx are not able to choose an audio device. I tried lots of different approaches (those that work on iOS and those that work on Mac for native apps) and I can list all available devices but I can not activate them.
So bottom line: only the current selected system audio device can be used to output sound. So make sure you setup the device you want to use in the Mac control panel. It that device is a multichannel device, you will be able to route to different outputs.
EDIT: Even if device selection would work, you would not be able to route audio to different audio devices at the same time. Not even Logic can do that. You can only work around this by creating an aggregate audio device. But this approach also works currently you just need to select that device as the system output device.