I mostly stick with 1 battery, either 18650 or a single 18350 in shorty mode. I've heard about needing to make sure the drain on both batteries are the same or else you can run into problems. If you're running 2x 18350 and one is draining faster than the other, it's possible you're getting some kind of resistance issue between them, which would cause heat. That position is right where the two batteries would be connected to each other... there's nothing else in that area that should be causing resistance issues (the only thing that'd cause heat).