I used a Blue Seas fuse block with pos & ground in the cabin and a bus bar near the batteries to complete circuits aft. Kept things pretty simple and easy to trouble shoot. The only long runs are the power and ground cables going to fuse block.
I really like this Blues Sea add-a-battery switch with automatic charging relay and starting isolator. Battery 1 starts and governs all engine functions & battery 2 is a Deka AGM that supports all other power needs. The upper part of fuse block is ground, the lower is pos. Both batteries are "on" and charge simultaneously while running, but they draw power independently of each other. Thus a bad battery can't drain a good one. If one happens to fail, you can still "combine" the circuits.
