Among other things, always keep a stack as a pair that is just used that way. Check the voltage of each battery after a charge and make sure they match. Rotate which battery goes into the "mod" each time they are used to even out wear, IE mark them individually and track usage.
Keep in mind, that if anything unexpected occurs, an 18350/800mah batter is capable of producing almost 40 watts of power, for a moment and there are two involved in a stack. That could be a little "disastrous" if it happens while you're holding the "mod" and vaping. Can anyone say "grenade"?
Now, I'm not saying anything will go wrong and I am far from an expert, but there is always the possibility. As an example, Boeing's recent experiences with battery fires.