Switch may be operated from battery ‘1’
to ‘BOTH’ and from ‘BOTH’ to battery ‘2’
or vice versa with engine(s) running.
Stop engine before switching to ‘OFF’
1 - it is my main battery
2 - spare - all electronics connected to this one
So I always start using 1. If it fails I go to off and try 2 (never happen)
When engine is working I always switch to both to charge both at the same time.
Shall I install anything more ?
Hard to imagine what happen when one be dead.