Hmmm yeah you're right rox. You might be too kg lol.
I kinda forgot to mention that I never let them past 3.7 volts.
The two batteries that you have used 5-6 times, will make no difference when you "marry them" as far as reliability and longevity of both batteries. At the end of the day, any battery can fail for no reason at all or do to unforeseen manufacturing flaws that can only be detected by engineers with proper equipment.
Its highly advisable, with any batteries, that you dont combine two or more that are of different age, and have been used for different purpose for a long period if time. Same way, different brands of alkaline batteries should not be used together and or of different age or make. In other words, when two or more batteries are used together, a younger/newer battery, will drain or put more(demand from it so to speak) load on the older battery/s.
In lament terms, think of it as running shoes that are years and or miles apart, worn out under different conditions unevenly.
In your case, two pair of shoes ran 5-6 miles on a different pair of feet or separate days. Just as long as they are same brand, make and size, you can pair them up.
Some Box mods use only one battery, so you can use that same battery on; mech mods and regulated mod that uses a single battery. Just dont combine that single battery with another battery on any device ones it has gone through an extensive cycle.
Its perfectly fine to marry your current two batteries, but ones they are married, do not un-marry them ever again. Unless its for a very short single test where a single battery is required. Example; Swapping batteries from a tv remote control just to make sure that a remote control works. TV Remote is an example of another mod.