One of the first things you need to decide is what function you really intend for them to do .
There will be a difference in management decisions between a "Starter" battery and a "Reserve" battery.
I, for example, have 2 4D's, paralleled, as the "Normal" bank. Used for everything.Then, I have a group 27, designated as a "Reserve" Bank.
This doesn't get used - only monitored for health - and is for if and when the "Normal bank fails to perform. I cant see ANY circumstance where my battery switch would be on BOTH.
The charger on my boat is not wired thru the battery switch - the charge sources go directly to the batteries - connection to the charger is not dependent on the 1-B-2 switch at all. And in fact, I have a on-battery disconnect switch on the Reserve battery, if I want to take it out of the charging circuits. I manage "one" bank- the "normal" one - and occasionally check the health of the "reserve" bank.
There are almost limitless options, and there is a trove of info on this site - have you checked out the "Electrical Systems 101" yet?