I agree with the above post. No need to monitor a pair in series assuming they are the same make, model, age, size, and type.
I have a high quality microprocessor 20 amp marine battery charger that would charge each battery separately. It would do that while the batteries are still wired in series and still under load. It would not display metrics on the two batteries.
You could use any good monitoring system set up for 12 volts. Add to it a $15 voltage monitor for each battery. The current through each battery individually will be identical since they are in series.
It is possible one could fail sooner than the other, but most likely the remaining one would be near failure as well.
I wish you good luck and happy trails ahead!