how is a 2-position battery switch wired?

[Tim.M]

I have two batteries and two engines, currently with one battery directly connected to each engine. Certain pumps and accessories are tasked with each battery which I don't like. I'd like both batteries to be able to power bilge pumps in case one battery fails, or to start either engine in case a battery fails.

I believe what I need to do is to add two battery switches that have a 1, 2, or All selection on each switch. What is the correct way to wire these:


Battery 1 <->\
Battery Switch <---------------------> Engine 1
Battery 2 <-------------->/

Battery 2 <->\
Battery Switch <---------------------> Engine 2
Battery 1 <-------------->/

or

/<---------------------> Engine 1
Battery 1 <-> Battery Switch
\<-------------------------------> Engine 2

/<---------------------> Engine 2
Battery 2 <-> Battery Switch
\<-------------------------------> Engine 1

I tried to make the lines approximate the length of wire required in each case to put the battery switch in an easy to reach spot.

[Tim.M]

Ok, I found one graph indicating the latter. The common on each switch goes to each engine starter. 1 on each switch goes to battery 1. 2 on each switch goes to battery 2. Position the switches close together and it takes no more wire than scenario one.

[Tim.M]

For early 1990's Mercruiser 350 5.7 liter engines, do I need Alternator field disconnect (AFD) on the battery switches?