Yep Double batteries some say are double trouble ..........its a bit like a man with two watches, he's never really sure what time it is
With double batteries
A) The alternator ( or Charger) will always want to charge the batteries with least resistants, normally the bank with highest voltage
B) Batteries need to have heavy blocking diodes between them so the charge sees them as seperate and so they dont drain backwards. A charge splitter that connects to the + outlet of alternator , that connects direct to battery and not rhrough the isolating switch.
C) Consider having a panel for each bank and keep them both fully charged
D) Use best lead HD terminals as we had failure in a brass clamp that was not even 12 m onths old . It simply fell apart. Lucky it was during land based service
E) Use silicone heat shrink silicone or other conductive silicone to combat sulphate build up and give best contact.clean terminals and wipe onto posts. Spray on is for show but doesnt combat corrosion unless its sprayed onto posts
F) In Marine apps make sure the crimps are soldered to stop internal corrosion . There are little solder pots avail online for $20.00 big enough to dip the whole terminal in .Tin both terminal and wire PRIOR TO ASSEMBLY Also heat shrink to keep out moisture once soldered
G) a weak battery will always pull a charged battery down ,
Batteries need to be bolted down, pay attention to detail in doing this as batteries hammer like hell in rough seas if allowed to. Check and torque terminals monthly . Use soft flexible cable, and tie or support cables near battery boxes , to reduce load on terminals
A roof mounted solar panel will keep batteries in top order. Also can charge GOOD batteries in half a hour or so , if some one has left a drain on
Hope this helps