I heard of some older cars where you just get 2 batteries in paralle arrangement and then your alternator just charges 2 batteries but the amount of amps it draws is from 2 batteries so the load is shared.

I think it was a VW Beetle I head that done with, done know how newer cars would work with it.

A couple of other cars have had dual batteries fitted, in at least one case because there wasnt enough room for a big enough single battery.

I got 2 in the Delica, just because I can

pffft, I used to have 2 min my first car... a 16 yr old browm fiesta !

just because... I could... tho they did help with starting too, the engine spun noticably faster