+1 about jeepers29 post.
Predicted battery life span is 3-4 years. Here in CA you can have a six year old battery that still starts the car. According to the owner "the battery is still good". Unfortunately, its not. "If" one battery took out the other battery, they were both bad to start with. Most likely the owner only replaced the one that totally failed.
The biggest issue I have about things people post in forums is, people give opinions which may not have any basis on fact. Its what they "think" is going on or their just parroting bad or partial information they read somewhere else. They also commonly blame what they see which is most likely just a symptom.
Just remember, the warmer your climate, the longer batteries appear to hold on. Back to the original question, "if" someone has a battery issue, is it actually the battery(s) that's the issue? The biggest killer of batteries that are under 4 years old is something pulling power with the key off. Also if a battery goes to zero, it pulls life off fast. The longer it stays at zero the weaker the battery is "If" you can even bring it back.