A multistage regualtor is a good idea for the house bank. I would not suggest a dual charging alternator or regualtor. You don't need that much. A combiner would do the job just fine and a isolator diode would be dirt cheap
if you compensated the voltage drop.
The muti stage regulator is going to add more amps when the house bank is low than when it is high. This is the charge acceptance based on how fiull the battery is based on the type of battery. Whan it gets near full you can not add a lot of amps. The combiner will steal from the house bank as it charges until it reaches full charge voltage. The starting battery should recharge quite quickly so it does not need over charging. The dual chargers are better since both are regulated but you just don't have that much battery to charge and they cost a lot more.
I prefer an external regulator because it can be smarter and the alternator can be dumber. The regualtor might last longer than the alternator though mine didn't. You save a little money
on the one but pay a little more on the other.
Your house bank will represent a big $$ investment and you want to be able to charge it as fast and as safe as it is to do so. You need to get a feel for how many amps you will burn between charges. Ideally the house bank never goes below 50% capacity and then you recharge it 100% for maximum lifespan (best investment).
Sometimes it really is all about the money
. Deep cycle batteries are what you need to live on and they cost a lot. For a fridge with lights and basic gadgets 400 amp hours capacity is a good-sized bank since you only use half ideally but could use more if required. That is the way real life is sometimes. In the end if you make the bank too big you'll never recharge it properly and it will die sooner from under charging. If you make it too small then you'll discharge below 50% on a regular basis and the batteries won't last as long. So how consistent can you be?
It all is perfect if you recharge everyday the amount you used the day before. You can do it any way that works by using less power (smaller bank) or making more charge (bigger bank). The regulator determines how fast you can do it given the largest alternator they make. The deal is if you oversize the bank the last 10% is a killer since it takes a lot longer. 10% of a large bank = can't get there normally. It's a case where too big a bank can be the worst alternative.
If you search around there is a LOT of details about electrical
systems on the forum in many threads.