The reason they are saying no is because most battery chargers are going to just pulse you're battery back to life. Pumping it with electricity flow until it reads high enough and even then it will just slow down to a trickle but it will still be charging.
You wouldn't want to stack batteries as then the same amount of
juice would be going through both and if they weren't exactly, I mean EXACTLY the same voltage, they one would overcharge, probably overheat, and then be bad news bears.
I'm sure someone could chime in with more technical terminology but that's the jist of it.