I've been using a different system for a number of years on my single engined boats.
A single "off-1-2-both" switch and two batteries. Battery A is a cranking duty battery, Battery B is a crank/deep cycle duty. Electronics are permanently wired to Battery B. This works for me since all electronics are unplugged and stowed in the locked cuddy. If one does not like this, then a seperate on/off switch could be wired from Battery B to the electronics. ( Although if you can't remember to shut off the individual electronics how could you expect yourself to remember to shut off the main switch?)

With this setup
1) I normally start and run the engine on Battery A with switch at "1".
2) Every second or third trip, I use switch position 2 so that Battery B gets recharged once in a while.

3) Battery B is available as an emergency "start the engine" battery should Battery A not be able to crank the engine.

Long days at drift or on anchor with electonics running will not discharge the main (engine start) battery...ever.

There have been times on some of my boats, that I used a simple charge splitter conected to the "off-1-2-both" switch to keep Battery B charged (well mostly charged).

ONE SHOULD ONLY USE THE "BOTH" POSITION IN AN EMERGENCY, where an emergency is defined as NEITHER battery alone will crank the engine. Keeping the switch in BOTH position under certain battery conditions can ruin the batteries.



, I NEVER use Both unless neither battery on it