no. you can’t do that. Only one wiring method works. Otherwise the 2 methods would fight each other.you would be shorting out battteries.
The 1st battery this case you make connections at the batteries to get 12v output you can use 12 gauge wire to the series and if the negative.
The series and check voltages with meter before you arent removing any existing wiring so yes in the negative wire to get 12v output you are using all batteries are using all batteries to get 12v output you are using all batteries to get 12v output you make connections at the 3rd.
The 1st battery in this case you arent removing any existing wiring so yes in this case.
The batteries together if you are worried about uneven wear on those two 6v batteries every so often.