If you forget to charge your second battery, don't use them until you do. You want a married pair, charging and discharging together. You really should get a 2-bay charger (at least) if you've got 2 battery mods, which you do. 4-Bay Nitecore chargers are like 20 bucks on amazon, and I assume a 2-bay is less.
Honestly if you've tried all these different configurations I would lean on something being wrong with the
tank. What that is, I don't know.