When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
The problem with only replacing one is that like he said the other may not be far behind, and the driver's side is temp monitored for charging so if the state of health of the batteries isn't close, you'll either over or under charge the passenger side battery.
When you add a new one to an old one. It will want to pull power to the older weaker battery to keep the system balanced. This will in turn kill the newer battery quicker. I replaced both on mine.