Guys,
I don't mean to speak for the original poster, but his use case is about as simple as can be. From his original post:
Background: We are the habit of turning on engine, motoring for 10 minutes or so and then sailing with selector to #2 (drain is limited autopilot, VHF and Navigation). At the end of the day, we start engine from #2, dock, plug in and all is well. On a recent adventure, we anchored overnight, running lights and water pump sparingly (all on #2). Two false anchor alarms forced us to restart the engine at night. In the morning, battery #2 read 10 volts and could not start the engine. We were fine with #1.
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So the boat is MOSTLY used for day sails. When not sailing, the boat is at a dock, connected to shore power, and has an onboard battery charger.
So, unless the OP starts making plans for a bunch of multi day trips that will be spent at anchor, there is a really no need for any changes to his electrical system. Even a single group 27 battery can provide enough power for a night at anchor. This would include the anchor light, some cabin lights, and perhaps a radio for entertainment. I do agree that combining two group 27 batteries into a single bank would be 'better' for the batteries, as long as there is a way of starting the engine if something happens and the two battery bank becomes disabled.
BTW, my last boat had 3 batteries onboard, 2 for the house bank and 1 for the engine start. They were connected to the standard 1 - 2- both - off switch. The onboard battery charger was connected to the same switch, so whichever battery was selected got the charge from the on board charger. So if I was in a slip with shore power, the batt switch would be set to 'both' and the battery charger would be on. When on my mooring, the switch would be set to 2 (house bank), or 'both' if the engine was running. At the end of the day, if I could not start the engine with '2' house bank I would switch to 1, start the engine, and then back to BOTH to charge all the batteries. This worked fine for the 8 years I owned that boat.
My new boat is much more complicated, with ACR, isolators, 7 batteries (1 for engine, 4 for house bank, 2 for bow thruster) and I am learning more about how all that works.
Barry