I read the other thread and can give you the answer to one question: Fountaine Pajot
apparently installed 2/0 wire on our 46ft cat., it seems they have a "Sunsail version" of the electrical installation
that we inherited when we bought the boat. It had only one small house battery on the starboard side.
We have since significantly modified the setup as per the attached circuit diagram. It is simple and works to our satisfaction, while reusing most of the pre-existing inside distribution.
We have only one engine battery for the starboard engine. The switch is there to add one for the port engine but in 5 years of cruising full time, we have never needed to use the engine battery and always start with the house battery (the engine battery is kept in good charge by the battery charger
during occasional marina stays).
Regarding wire thickness, we have 9m wire between the 2 house battery banks. The original wires are 2/0 and I have doubled the ground connection with a 1/0 wire.