It is recommended that batteries be replaced in pairs.

You CAN replace them separately and you might get away with it, but your batteries are connected directly to each other with no protective circuitry of any kind. If there are differences in the batteries (like float voltage or internal resistance) one will end up trying to 'charge' the other one to the detriment of both. Since the batteries are hard-wired, this happens even when the key is off.

Save yourself a world of grief - replace them both at the same time. If you have one 'good' old battery, install it in another vehicle. I put my best 'old' battery in my tractor.

Thanks to wow.