Vincent, I may be misunderstanding your setup, but there may still be a common link, given that receivers share a common ground strip - hence the common ground problem when charging two batteries connected to the Rx at the same time.

JohnMac, I didn't know one could use a diode and two connections to two Rx ports. If I understand correctly, the BattShare, the PowerBox Sensor switch and the other similar devices all may use two switches, but have only one connection to the Rx. Am I missing another setup possibility, namely one with a diode and two Rx ports (?). As to the case you've mentioned, it is indeed quite disturbing. On the other hand, A123's handle parallel installation mishaps differently from Nixx. They take much longer to drain into each other.

Thank you both for enriching the discussion. I believe that this is one critical aspect of our installs that still needs much clarification...