I am thinking of adding a 24-volt trolling motor in the future. This
means I need to change the current batteries settings.

Currently, I have these two batteries:
(1) A 12-volt dual-purposes flood battery (group-24).
(2) A 12-volt deep-cycle flood battery (group-24) (the manufacturer
spec also says that it can also provide service for starting; sound
like its main purpose is to serve as a deep cycle battery; but I can
also use it for starting motor).
And I also have a battery switch that has 4 positions (1, 2, BOTH,
OFF).

Seem like one way is to add a third 12-volt deep cycle battery and run
this battery with the existing deep cycle battery in serie. This
should give me 24-volt -- just for the trolling motor. And I
"probably" doesn't need to change the existing battery switch. But I
am wondering if this will force me to put all the other electronic
equipments plus the motor-starting task to the one dual purpose
battery. This doesn't sound right. Am I missing something. Moreover,
does this also mean that I need to replace

The other way that may seem to work is to leave the existing batteries
and battery-switch alone and add a 24-volt battery (is there such a
thing?) just for the 24-volt trolling motor, and then use a
12/12/24-volt battery recharger. I guess this means I need to replace
the existing battery switch, right? Or should I leave the existing
battery switch alone and add a new ON/OFF switch just for the 24-volt
battery? The problem is: How does the alternator charge the 24-volt
battery anyway? Does this mean that I am better off replacing the
battery switch?

Is there any other better way? What's the best way?

Thanks in advance for any info.

Jay Chan