As usual, Mainesail has a
great write up on what ACR are and how they work. I strongly suggest you read that article. In short they are voltage sensative relays. If either battery has a charge voltage over 13 volts for 90 seconds or 13.6 for 30 seconds the ACR puts the batteries in parallel just like turning your battery switch to BOTH.
So if you have solar feeding the 4D, once the voltage is above 13 they will be combined and charging. When you plug into shore power, same thing. And when you run the alternator, again the same. So if you are on a mooring, when the sun gets high enough to start pumping some good solar into the 4D, it will combine the batteries and start charging the group 31 as well. When the sun goes down it will disconnect the batteries. Its just like if you were on the boat and turned it to both when you saw the solar was charging and back to Bank 1 when it was not.
The factory setup for your alternator is that the charge goes through the battery switch. So if Bank 1 is selected, it goes there. The ACR would then combine the banks and you would charge both. But you could make the charging more efficient by having the alternator output run directly to Bank 1 and adding a ground connected to your grounding lug instead of relying on grounding through the body of the alternator. But this can be a future upgrade.
Another consideration is the shore power charger. I would personally disconnect the line running from the charger to Bank 2 (Group 31) and just let the ACR handle the charging of that Bank. This wire could then be repurposed for your feed from the MPPT to Bank 1.
Now one thing to consider is that you may not want them to have the same charge profiles. Even if both batteries are the same chemistry, they might not have the same bulk, absorbtion and float voltages. In this case the Sterling Battery to Battery charger might be a better option. Mainesail has an
article about that too.
Back to your main question. If you use the same size wire as the feeds to the banks, 2/0 AWG, then it is fine to use the same ANL fuse as you already installed.
On the solar controller, if you have not purchased it already or you can return it, I would consider spending a few dollars more and getting the Vicrton SmartSolar 100/20 (not the BlueSolar, the SmartSolar has built in Bluetooth). Not as good as some others like Midnight KID or Outback but much better than the majority of the cheap ones on Amazon.
Good luck,
Jesse