I own one of these Fender Mustang amps. Never thought you could record through the USB cable. I just read the documentation for it, and sure enough, it says I can do that!
WAY kewl!
I am fairly certain that you can have multiple interfaces active in Sonar, other threads have discussed that, the one that comes to mind was talking about setting the Record Latency settings per device.
So, it would seem to me that you would just pick the device for that audio input and be on your way.
PLEASE let me know if this works for you, as I would carve out some time to work with that.
Bob Bone