When you use a mod with more than one battery, it is important that they are paired. This means that you use two batteries of the same brand, same chemistry, which are bought together, used and charged together. As long as you are using the paired ones as they grow old together, which means that its internal chemical balance are changed identical during time.
Batteries used for e-cigarettes often squeezed to the limit. If you use batteries with different chemistry (even same brand, but used in different ways/time span), there is a risk that one of the batteries are pushed over their limit making its chemistry unbalanced. An unbalanced battery has the potential to be dangerous, and we do not want to risk anything unnecessarily.