The difference is pretty simple. Since you have more than one battery,
an onboard charger is the way to go. It will charge, monitor, and
maintain each battery as if each had it's own charger.

Sound good to me. Seem like an onboard charger is the way to go.

You can also buy models that handle one 12V and one 24V
at the same time (or other configurations) that are common
on bass boats that have a 12V starting battery and a 24V or
36V deep cycle trolling motor battery.

This sounds odd. If they only have one battery for starting engine,
and another 24V or 36V battery for the trolling motor. Then all the
other electronic equipments (such as fish-finder, VHF radio) may have
to connect to the 12V battery that is supposed for starting engine.
This doesn't sound like something that I would want to do.

The other thing is that I cannot find any 24V battery. Seem like
people tend to use two 12V batteries in serie to make a 24V system.
This kind of make sense. Then we can use those two 12V batteries as
backup just in case the starting battery and the house battery both go
dead. This is just that two 12V batteries may be a bit too heavy and
take too much room. I am just hoping that I can find one single 24V
battery, and I am still looking.

I would never charge two batteries with a conventional battery charger
at the same time. They are never the same so while on is starving for a
charge, the other may overheat - or worse.

Good to know that the onboard charger has this benefit. Thanks.

I have decided to get an onboard charger tomorrow. The onboard charger
will charge those two existing 12V batteries. In the future when I get
a 24V trolling motor and two 12V batteries for the trolling motor, I
will get another onboard charger to charge those two new trolling motor
batteries. This sounds like a plan.

Jay Chan