Basically the Automatic Charging Relay allows you to have two batteries, one for the engine and one for the house, and keep them isolated from each other (that way you cant accidentally kill the engine start) but also allow them to each charge independently from one charging source.
Throw away the 1-2-All battery switch and get an on-off-combine switch and the relay.
This kit will do that for you.
https://www.bluesea.com/products/764...tery_Kit_-_65A