Thanks for the additional details. Now we're getting somewhere :-)
I know about Admirals and Xmas....mine has set some chores for me, too. And, it being Xmas Eve, my head may not be as clear as it could be :-)
If you bear with me, we'll sort this out over the next few days. There are a couple of other items I need to know, but some things are clear from the information already provided.
From what you've described so far, it seems the major concern to be overcome -- and it can be -- is the long run from the house battery bank to the charging sources.
Since you're planning to essentially remove all the old wiring and start anew, that really simplifies things.
I assume you'll want to stick with a group 27 AGM for the start battery. It's generally best to stick with the same type of battery for both the house and the start batteries.
Where are you planning to locate the start battery? Will it be aft near the engine compartment, or forward co-located with the house batteries under the settee, or elsewhere?
, you'll want to treat both group 27 house batteries as a single bank, i.e., run them in parallel.
You'll also want to run all onboard charging sources...the battery charger
, the alternator, and any others like solar or wind or diesel generator directly to the house batteries, with appropriate fusing located close to the house batteries. "Appropriate fusing" in your case will mean ANLs, since you don't have room for the MRBF terminal types.
You'll want a healthy sized cable going to the house batteries from the alternator output lug, and another same size cable running from the alternator ground to the house battery ground lug. Sizing of these cables will depend both on the size of your alternator -- present and anticipated -- and the length of the cables.
I believe it would be good in your case to use an EchoCharge device to maintain the start battery. This is a little device which sits between the house batteries and the start batteries and maintains the start battery automatically. Nothing to switch, and nothing to worry about once it's installed correctly. The only tricky part is getting the wire size correct, and that depends on the length of the run. Since the maximum current carried by the EchoCharge is 15A, you don't need a huge cable as you might with a "combiner". The device can be located anywhere it's convenient, generally better outside the engine compartment and outside the battery compartments. I like to put them where you can see them.
You may well want to install a safety fuse in the starter
cable as well, even though ABYC doesn't require one. It's a very good idea to have one, properly sized, to avoid a catastrophic fire in the event of a solenoid or starter
We can leave the wiring of the 1-2-Both-Off switch for another time, but I generally like to do something like MaineSail shows in his diagram, i.e., use the #2 position only in an emergency when you need to start the engine from the house batteries. I know Maine likes to use house batteries for starting, but I don't, both because they're not designed for it and because of the possibility of unwanted transients to/from other equipment on the circuit. Much better, IMHO, to keep the start circuit separate, and to use the start battery to start the engine as it's built to do. Of course, with AGMs there's not all that much difference, if any, since they are very capable of putting out a lot of current quickly.
And, if you wire it like Maine's diagram you can do either: (1) keep the start circuit separate, using it's own ON-OFF switch, or (2) keep it really simple by leaving the switch in the BOTH position and starting the engine from the house batteries. For some folks this is easier; Maine thinks some may not be able to handle the concept of a separate ON-OFF switch for the start circuit :-)
AGMs are capable of soaking up a lot of current when charging, easily taking their AH rating in charging amps. Two Group 27s would give you roughly 200AH and it's highly unlikely you'll have anything aboard capable of generating that much current. However, unless you're careful, the AGMs could destroy a small alternator which isn't designed to put out it's maximum power for hours on end. You might tell us what you're thinking/planning re: alternator/regulator situation and upgrades, if any.
Enough for now. It's Christmas Eve.
Very best to you, and to all who follow this Board...