Looks simple enough and there are many sample wiring diagrams available but I have what is probably a noddy question.
On my existing setup I have a black cable that runs to earth on the engine.
I have a heavy red cable that runs to the isolator switch.
I also have two thinner cables off the positive side that I don't know where they run to. What would they be for and how do I accommodate those in a twin battery setup, or do I just leave them on one battery and use that as the house one and wire the other battery in for starting only.
Are they, for instance, a feed for items that are always live, bilge pump / CO monitor for instance?