Sorry, not sure I understand the question. Can you explain more clearly? 

Line in input has nothing to do with Bluetooth input. They’re separate. 

If you have two ERA 300’s “bonded” as a stereo pair, a stereo music source connected to the USB adapter will play in stereo using the two speakers. The connection between the paired speakers is via WiFi, not Bluetooth. This stereo source can be played in any combination of SONOS Rooms.

Thank you much