I think it's because it's a USB signal transfer and not with an audio jack via an interface. But you can try to distribute the microphones to two tracks via Garage Band, maybe that will work. But otherwise the system doesn't understand that sound could come from two inputs. Otherwise, I advise you to work with an audio interface and appropriate microphones, then you can control the inputs much easier. But maybe I'm talking complete nonsense, I'm no expert :)