Don't know if this description will be of any help or not......
About a year after we bought the boat in 1998 we had the original "dumb" charger replaced with an inverter/charger. At that time we had three batteries to charge: 2 8Ds and a 4D. The 8Ds served as house and start and the 4D was dedicated to the generator.
So we wanted the capability of charging all three (but separate) batteries when we were on ground power or the generator.
On the advice of the proprietor of the marine electric shop we use, we had him install a Heart Freedom 25. At the time, this was the smallest inverter/charger that had two echo chargers.
The main "leg" of the smart charger were connected to one of the 8Ds, the one we used for house power. The two echo chargers were connected to the other 8D (start) and the generator battery. The echo chargers provide only the third stage of the smart charger, which is the so-called "float" charge. This will charge a battery at a much slower rate than the three-stage smart charger and then hold them at a full charge. This "echo charge" is sufficient for the start and generator battery.
This arrangement has worked extremely well. More recently we replaced the two 8Ds with six, 6vdc golf cart batteries, threee in each 8D box. Two of the GCBs have replaced the 8D start battery, and provide roughly the same 100 ah power.
Four of the GCBs replaced the 8D house battery, and provide about 200 ah, roughly twice the capacity of the previous 8D.
The charger connections remain the same--- the full smart charge connection to the four GCB house bank, one echo charge connection to the two GCB start bank, and the other echo charger to the generator's 4D battery.
Were we doing all this today, we'd probably use the same setup with the exception of the brand of inverter/charger. Ours is a Heart from the days when they were an independent company with a great reputation. This repuation went considerably downhill after Heart was acquired by Xantrex. So we would most likely look for a more reputable and reliable brand of inverter/charger.