It is generally ok to stack batteries, but it is concidered to be more of a "risk". The reason is,especially with as much draw we put onto a battery while vaping, that one battery in the stack will tend to drain faster than the other one, so there is a possibility for one of the batteries to discharge past the suggested 20% min charge threshhold (not 100% sure on the LifePOs will get back to that later). Another thing that one should keep in mind while using stacked batteries, is to keep the batteries in pairs, meaning that always charge and use the same two batteries as a set.
Vaporers have been using stacked batteries since the beginning of modding with no issues at all (Including me) but there is more care needed for stacked than single batteries.
LifePo,s need to be charged with the charger that is meant for it, that,s onething that I have been hearing since the use of them.
LGD