That parallel charge circuit does not appear to be a typical installation and I think that most of us are unfamiliar with it. I think it's pretty clear that the reason both battery banks drained is because it was on/closed, which combined the banks. The point everybody is making is that the two banks should normally be isolated. There are several ways to install switches to provide both isolation and redundancy. There are better ways to charge the batteries without combining the banks. It's up to Nick to determine how he wants to do it.
