Agree with the above. There is no rational reason to stack two 18350 batteries in place of a single 18650 battery. You don't gain more battery time (mahs don't double), in fact you lose it.
Two 800 mah batteries = just 800 mah total. A single 18650 battery will give you 2000 - 2900 mah in just one battery, depending upon the model.
You didn't say which "Segelei V2" you have. If you're talking about a ZMax, you gain very little additional performance by stacking; at least not enough to warrant the additional risks involved by stacking.
If you don't follow a specific ritual of stacking, one of the batteries will become weaker than the other over time. The weaker battery will be much more likely to be over-discharged and go into failure. One failed battery will cause the second one to go into failure, and then you have the recipe for a battery catastrophe.
When stacking, the batteries must be matched as a pair when new, be the exact same model, and be equal in performance. They must be
alternated in the order that they are placed in the mod (mark them for easy identification to tell them apart). The voltage of the batteries must be monitored both coming off the charger and coming out of the mod.
Statistically, most battery incidents occur while they are charging. High quality batteries deserve a high quality charger. Both are equally important to a good safe setup. Don't skimp on a cheap charger.
