Do a search on here for
battery combiners, chargers, and such. There have been hundreds of threads on this, many very recent (as in last couple of weeks). There are pros and cons to the different ways to do it.
It goes beyond just
charging a start and house battery design. It really should be a "holistic" design that includes: which battery will be charged by the
alternator (house or start), type of switches, Echo-Charger/Duo Charger/other dc to dc chargers, types of batteries if they are different, etc.
The solution may be simple but the decisions are not trivial.
It's not clear to me how an
inverter is part of your question though for an
outboard boat on the
water. You can use an inverter/charger to charge batteries while on the
dock with
shore power but it won't do anything for you on the
water. Your outboard will have an
alternator that usually goes to a start battery. An echo
charger may be all you need or a 1-2-all switch. I'm not a big fan of 1-2-all switches for what you want to do. It is too easy to leave it on "all" while you are running the
motor and then both batteries will run down at the same time if your
charging fails. A duo
charger can do a better job of charging two different types of batteries.
But if you are very careful and isolate your batteries after charging them both up, a 1-2-all switch is the simplest way to go.
Look at the Blue Sea site. It has some configurations but remember they are trying to get you to buy their products not necessarily give you what is best for you. They do
work though. Here is one product they sell:
https://www.bluesea.com/products/764...tery_Kit_-_65A
Here is their
wiring diagram:
https://www.bluesea.com/systems/42/2...ectical_System
But this system uses a battery combiner and there are pros and cons to that setup.