Settled on my dual battery switch circuit setup

Normal running is put both switches on #1, #2 is normally not connected.

This design lets me
1. Swap banks, put both switches on #2, port battery then runs house buss, starboard runs starter buss
2. Turn off either battery, isolate yet keep alternators connected
3. Isolate either bank, and let either bank run both sides, both busses.
a. Turnoff starter bank and let house bank run all, left switch off, right switch on 1+2
b. Turnoff house bank and let starter bank run all, left switch on 1+2, right switch off
4. Combine banks, put either switch on 1+2, with other switch left on #1

This circuit design makes sense to me, and gives me a lot of options.
Putting the battery direct to the C (common) lug, allows for this.

 

Forgot to put my battery bank combining relay across the common lugs on the battery switches.
Mine is a continuous duty 80 amp golf cart relay, others are known as ACR. I run mine from ignition on and a manual on-off switch.

 

Got it done today, had to swap some big wires around to make it all fit without making new wires.
IT works good, is cool to be able to swap banks by putting both switches on #2.
Next week will hook up my ANL starter bank fuse.
Still waiting on my Camco double battery box from Amazon.

And going to move my battery combining relay wires to the ANL fuse versus attached to the common lug of the switches.

 

I found when I wired up everything, I preferred to have my combining relay joining from starter bus to house buss, not on the switches. The switches were too crowded with big wires and it looked messier. Functionally it does the same thing, plus the wires then are sitting nice and flat on the back wall, along with the starter wires and house buss and inverter wires.

 

Hi....the starter/alternator link, steerage buss supply and enhancers should all tap the normal post of the double battery switch. Nothing ought to go straightforwardly to the battery up-sides with the exception of the '1' and '2' switch posts, a memory lead in the event that you decide and a bilge siphon/drift switch when relevant. There's only one acceptable way.

As a side-effect, one day when you change your batteries, in case they are both profound cycle and balanced (normally fine in warm climate with a little square), you can substitute the battery capacities with each excursion and increment the life span of your batteries

 

https://photos.app.goo.gl/VfXSFDv7jDpos1KV6
Link to my mostly final setup.
The fuse works, engines crank fine. The ANL 300 amp fuse does not get hot at all, I felt it with finger after multiple crank tests.

Cranking video here
https://photos.app.goo.gl/3XAgajrV9F8WxMk88

This is an Attwood battery box, and so far I can not figure out the strap buckle.
Can anyone tell me how to thread the strap? Maybe with explicit photos?
And the buckle has separated into 2 pieces.

Batteries are well blocked cant move more than 1/4 inch. I went with a SS strap for a hold down, and it is all on the negative side of both GC15 golf cart batteries, cant really short, plus it is taped over. I gave up on the idea of the Camco double battery box, dont really need it! But if it ever gets shipped to me, maybe will try it, I just dont know.

 

got the solution
Help, I Cant figure out the Attwood battery box strap buckle - The Hull Truth - Boating and Fishing Forum

 

Looks pretty good. Still not understanding fusing the cranking side.

 

I thought I should after including the generator in the start bank circuit. It is a long wire of about 15 feet, well protected, runs to the aft lazarette compartment, but I kept thinking what if something happens to it, could burn the boat up. Most likely nothing will happen, then I saw on amazon the fuse holder as a cheap warehouse item, so got it for $17 (actually 0$ with points) and two fuses I paid $14. So the cost was low.

And I watched the Compass Marine's video of him testing 300 amp fuses with a powerful lithium battery bank. They instantly blow on dead shorts even using only 8 gauge wires. So even much smaller wires than rated for a 300 amp fuse are protected against shorts. My smallest wire is very short length of 4 gauge in the battery circuit on the number 2 position which will likely never be used. Most of the wires are 2/0, the generator has a 2 gauge wire, but when I miced it out, it seems more like gauge 0, it is made by Standard Motor Products and came out of a mid 60's boat, wire has a blue rubber cover and was used for starting that boats v8 engine.

Your not carrying continuous high current flows on starter wires.
Since it is a start circuit, the power demand is of course temporary, so a dead short is what you mostly want protection for, not wire overloading.

Anyhow that is my POV.
Check out this video
Marine Rated Battery Fuse Trip - YouTube

 

ok, here is how the strap works pictures. It is easily cinched down by pulling the strap tail, and easily loosened by lifting up.
And it is not intuitive at all. What fools you are these hook structures that seems like they should lock in, but all they do is angle the clasp upwards to aid in having the teeth engage the strap. Posting this as maybe it can help someone else.