Stacking to me, as we used the term 5 or 6 years ago, means putting the batteries in series. With two 3.7 volt batteries, which are 4.2 volts at maximum charge, you would get 8.4 volts to the atomizer (Not accounting for voltage drop.). That would be something like 141 watts if you screwed an Atlantis or Subtank on there, so a lot of power.
The main problem people experienced with stacked batteries in series was one battery discharging faster than the other. This could result in the device still producing enough power to vape while one of the batteries was below its minimum voltage. I'm not sure how modern batteries react to being over discharged, but back then it could cause them to become unstable and vent.