An extra battery merely extends the battery reserve time which then has to be replaced (recharged).
If it gets thru the day so that it can recharge at night, then fine.

Without knowing the alternator profile (ie, driving times at varying RPM and hence its output) and the other loads, whether it "will work" is uncertain.

And as usual, the only way to know the condition of an alternator and battery system is with a voltmeter (ie, if the alternator ages & reduces output, or the batteries age, or some other charging fault).