Although it isn't wrong to replace both at the same time, I disagree with this statement, especially the word "need".
Batteries or other technical systems are like living beings: even if apparently identical, they die at very different moments...
- they are used in a different way, for instance the Aux "never" starts up the car
- they were produced at a slightly different time, with very slight different materials, very slightly, I admit
- the quality of the materials are therefore slightly different.
- there could be a significant "mistake" or "contamination" in either one that could shorten it's life significantly
Conclusion: it could well be that one battery dies after two years, while the other lasts 5 years.
In my case, if one battery dies after two years, I will keep the other.
If one dies after 5 years, I will probably replace both!