Slightly different, but related question: My bike has the wireless unit installed in the stoker seat tube underneath the battery, although we are not using any wireless capability at present - i.e. no Garmin. It also has a Shimano Information Display which supposedly also has wireless capability. The Info Display was added to replace the A junction under the stem. I've been told that the system won't work if there are two wireless-capable nodes in the system, and yet it seems to work just fine.
True or false?
I suspect that the real answer is that the system will shift just fine, but will have trouble connecting wirelessly to a laptop or other Bluetooth device.