When you choose to stack batteries, one battery always drains faster than the other. If they are continuously put in the mod in the same order (and not alternated in the order), one battery gradually becomes weaker than the other batter and becomes a "weak link" in the pair. That battery's probability to fail and go into thermal runaway increases in likelihood as the battery ages over time.
"Stacked or unstacked?
In some mods, you can fit two 18350's when the mod is configured for an 18650. This boosts the voltage output from 3.6V to 7.2V which means more vapor. Yay. BUT, you should NEVER do this. Unless you are familiar with matching batteries, you are only endangering yourself and others. Sometimes, just because you "can" doesn't mean you "should".
You may think, "Well, shucks, laptops have multiple batteries in them, why can't my e-cig?" Laptop batteries are specifically designed for stacking and are paired for use at the factory. When using un-matched batteries, they will discharge and provide current at different rates. This means one battery will continuously take more of the stress than the other.
You may not notice any trouble at the start, but as the batteries age with use the problem will become more severe. Push the batteries a bit further and now you've entered the realm of thermal runaway (see pics at beginning of this blog). Downside being that one battery entering thermal runaway will push the other battery into thermal runaway. Now you have two small, flaming explosives only inches from your face. Save your teeth and home, don't stack batteries." --
Deeper Understanding of Mod Batteries (pt II)
