adjust your assumption:
the gas tank is empty at 2.7v
it is half empty at 3.6v
read
Lithium Ion Battery Safety 101
test
use your light until you notice it get dim, confirm the battery voltage on a meter at that point
now a warning
using a light with Two Batteries has a additional risk, IF they are not of matched charge. It is possible for the battery with higher charge, to transfer power into the less charged battery. This is dangerous. Therefore, I would not recommend a light with two batteries, to a newbie, unless they are of equal charge, age, condition. Dont mix and match batteries with different charge levels.
battery chemistry
Unprotected ICR is most dangerous
Unprotected IMR and INR is less dangerous