Stacking the 18350 would yield 7.4 volts (each 18350 is 3.7 volts). This will yield a higher wattage across the same resistance, but at high drain, they would heat up very fast. Add in the extra connection (battery to battery) and you could end up with more problems. A number of videos that I have watched have shyed away from that setup as potentially dangerous. YMMV
I don't know why FastTech does that.I guess the reason why I asked was because the description on this KTS mod stated that 2 18350 can fit in this mod, which led me to believe that stacking was normal for this mod.
They also sell unprotected Li Ion batteries alongside protected Li Ion and IMR Li Mn batteries. Make sure when purchasing batteries from FT that they are either protected or IMR. Unprotected Li Ion batteries have NO PLACE in vaping.Contrary to a previous comment, stacking two batteries will decrease the 'drain' (current consumption) by nominally half for a given power output at the coil...assuming the mod is built to accommodate the higher voltage supplied.
Connecting two batteries in series does not double the available capacity in terms of mAH, rather that remains the same and the Voltage is doubled. With double the voltage, the available power is increased. The higher voltage makes available more potential power and allows operation at lower current rates (drain). Higher Voltage allows for use of higher resistance which also which effectively lowers the drain.