If the mod supports it you can. I think people were giving some bad advice or didn't understand the question. Lots of mods can be vaped while charging. Generally speaking it's better to charge with an external charger because it's faster/better but that's up to the user. Nothing wrong with charging while vaping or vice versa
The advice isnt bad and the question was both understood and answered perfectly in the first reply.
We SHOULDN'T charge 2 or more cells in any mod, they should be charged in a quality stand alone charger. This is not saying that charging cant be done in a mode, but most of them do a TERRIBLE job of balancing the charge. It is a better recommendation not to charge in the mod period.
If charging in the mod is unavoidable, then you SHOULDN'T disrupt the charging cycle which is EXACTLY what happens when you take a draw. This puts added stress on the cell in the middle of charging. In fact it is recommended that before charging your batteries you give them about a half an hour after their last use before putting them on a charger. This allows the internal chemistry of the battery return to a less volatile state before charging.
This does not mean that pass through charging CAN'T be used and that you CAN'T vape while charging, but the far better advice for the safety and longevity of your batteries is not to.