I must start by first stating that I know every battery manufacturer says not to stack them. I also know that it is done by quite a few. I received 2 18350s yesterday and just to see what the fuss was about I stacked them and took one puff just to see the difference. I was amazed at the quantity of vapor(and heat) that came out. I then immediately took the setup apart and will do more research before I try it again. If I were to do this I know that the batteries have to be pretty well matched as far as you don't want one to have a full 4.2 volt charge and the other to have a 3.9 volt charge. I would also assume the guys doing this are building higher resistance coils for this. What are your thoughts? One other question, I know the amp rating of the efests are 6.4 amps, does stacking them increase the amperage limit or does it remain the same?