If you lived here, you'd run an isolator if you had two batteries. Even the diesel guys run isolators. It's better to have one battery dead instead of two. It sure would suck when an oxidized plate loses a chunk and shorts a cell and kills both batteries.
My Dad had a diesel car that he parted out 15 years ago. It had a solenoid between the batteries with two fusable links coming off it going to the glow plugs. I was going to use it on my truck but I went and bought a Ford solenoid for $3.
I just drew up in PSpice how I have my dual batteries wired. It looks kind of crappy because it's a jpeg but it'll have to suffice.
There are zero splices. All connections are done with bolt-through ends that have been crimped and then soldered on. It is very important to solder everything. If I had a dollar for every rat's nest I've fixed that had shoddy splices and crimp on only connectors I'd have enough to buy me a winch. It's best to shrink tube everything but I don't have large shrink tube and only use electrical tape. Remember, when wrapping with electrical tape the thickness of the wrap should be NO LESS than the original insulator.
You can make your own custom cable ends for large diameter wire using copper pipe. Find copper pipe that fits relatively snugly over the copper conductor. Buy a couple feet of it. Purchase some 60/40 solder if you don't have any. Cut off a 2" piece and crimp about 1" of it in a vice so it is perfectly flat. Drill a hole in the flat spot the diameter of the fastener you're going to use. Remember, the rule of thumb is to have close to as much copper left on each side of the hole equal to the radius of the hole you drilled. Now drill a small hole (1/8" will work) into the uncrushed part of the terminal but close to the crushed part. This will be used to fill the terminal with solder. Now stick the wire into the terminal all the way and use a punch to crush the pipe. I usually only use one punch mark but some people do another punch mark 180° away. Now use a propane torch and fill the terminal with solder. Don't forget to tin any leftover exposed copper.
There are two main takeoffs at the positive terminal of the main battery: #1 The 2 gauge wire that comes off the battery to go to the solenoid then to the auxilary battery. #2 The 4 or 2 gauge wire that goes down to the starter.
At the starter, there are now three wires connected to the large positive terminal: #1 The 4 or 2 gauge wire that comes from the main battery. #2 The 10 gauge or larger wire that goes to the alternator. #3 The 10 gauge wire that goes to the fuse that goes to the battery isolator that goes to the auxilary battery.
