If U have a car there , take the batt out and one at a time replace the deuce batts and U will then find out if one is bad trial and error, this way U do not need a tester u are using a batt that starts a car as the tester, U do not need to remove the middle leg to charge em, even if u only use one batt on one the charge will filter over to the other Charging a batt is like filling a pool, to prove this drive your truck and think about the alt , there is only one and it is charging both, like i said do not make it complicated, as long as u do not put more than 12 volts into a batt u are doing it right. less water takes longer, like a longer jump cable sure there is some drop, Just be patient it will do the job, And u can hook a 12 volt off either batt, but there are consequences, if one batt draws down the other will want to stay the same voltage, when batts are not fully charged they sulphate and there life is shortened,, It is 100 % human error on almost all batt failure. not maintaning them in a clean and fully charged state shortens there life, If i were to take 12 volts from a batt I would take it from the batt that is taking the charge from the alt, and even then pull them and clean them and charge them individually every 4 to 6 months and rotate them.