I've just been trying to find some sources for this. I can really find very little.
Maybe I'm talking out of my arse.
As I understood it - connecting batteries in parallel and allowing them to rest for long periods of time is a bad thing. It's OK,if the batteries are identical, or they are under constant charge or discharge - i.e. connected to a solar power rig. Yet I can't find much to back this up.
I had a Landcruiser with twin batteries. One was disconnected without the engine running. When cranking they were connected in series to provide 24V for the starter.
if one battery looses charge faster than the other then the bad one will discharge the better one - they all lose a bit of charge over time.... but it's not a problem unless one battery is pretty much knackered
and if one battery is much better than the other, then that'll supply more power than the other when you put a drain on them starter/lights/whatever
but.. giving them a full charge regularly pretty much solves all the problems, they get nicely balanced out... alternator puts out a bit over 14v which is more than enough
if you're living off grid or somewhere on battery power you don't normally give them a proper full charge (not to 100% anyway) so you don't balance them out the same, if one battery is lagging behind the other, and then it's not getting a full charge when you charge/discharge cycle them you end up with one miles behind the other
