Batteries age with a fairly predicatable life of 4 to 5 years. Some external circumstances will cause a battery to fail prematurly (heat,vibration,and recharging rates)
Consider this fact: Even though two batteries are manufactured near the same time,then installed at the same time....the conditions within the installed vehicle for two batteries ARE NOT THE SAME.
The gutter side of most vehicles usually encounters more potholes and road impacts than the drivers side,especially if you live in an area that freezes in the winter.
The engine compartment does not always have equal ventilation on both sides of the engine compartment.
The battery cables are not always equal length or condition. You can have a situation
where one battery side repeatedly fails before the other.
In your circumstance,you could go ahead and replace both batteries purely out of conveniance. Chances are pretty good that the second will die within the next year or so.Its fairly predictable.
IT IS NOT NECESSARY THOUGH. You can determine with simple and inexpensive tools what the state of your batteries and chargeing system are tho...even if you decide to replace both anyhow,You should always do three simple tasks before replaceing.
Check and clean your battery terminals
Check the voltage chargeing the batteries for 13.6 to 14.3 with a quality voltmeter.
Check the battery cells with a two dollar battery hydrometer.
Optional checks: IS YOUR battery holder properly installed? Vibrations kill lead acid batteries prematurely.
Are any heat sheilds missing from your battery? Heat can kill batteries prematurely.
Are your shocks ok? Vibrations kill lead acid batteries prematurely.
If you dont do these checks...there is a good chance you will be back in this same situation sooner than you like...even if you replace both batteries.
If I were in your fiscal situation. and I have been in those shoes before....I would find someone that insists on replaceing both batteries at a time no matter what ,and say ,hey trade me that one good battery for this definately bad battery .
Anybody out there near the OP in that situation, about to swap them both out???