Advantage of installing two 12-volt batteries

It all depends on how you wire it into your system. What get cut-off when the car is running and what stays alive when the car is off. Does both batteries charge while your car is running?

 

Some people put two 12V batteries in their vehicles for extra cranking amps, which doesn't apply to any Toyota hybrid since the engine is started by the high voltage battery.

Others put two or more 12V batteries for extra battery energy with the engine off. A use case scenario in a car like a Prius could be a GMRS radio or Amateur radio used for transmitting a signal.

Another is to have extra power for something like a very powerful stereo amplifier. The second battery could be a high power type, such as LTO.

Another reason is for redundancy, which there are a few different ways you could tie in a redundant battery depending on what you're afraid is going to go out. But on a Prius you're not going to worry about an alternator going out since there is no alternator.

 

In reality it will just add weight and take more energy to keep it charged up. Much better to have a hundred dollar lithium jump pack accessible without opening the hatch. When the day comes your 12v battery is discharged due to age or lights left on, you pop the hood and safely jump pack it. If someone asks for a jump you pull out the lithium jump pack. Either way, you are a hero rather than a zero, which could happen if jumper cables were reversed and your car has to be towed.

 

No point in two batteries

 

I'd recommend supercapacitor jumper packs over anything with a battery, even lithium. There's no possibility of it dying an early life from selfdischarge or from heat or cold. The down side is if your battery is 100% completely dead and don't have anyone around with a working battery then a supercapacitor jumper won't work. The other downside is if you need it for supplying power with a failed alternator.