Recharging is done through a connector which is supplied with the unit for this purpose, and which connects to the main cylinder valve to decant air for a top-up.
Independent twin cylinders, (Two cylinders of about the same capacity mounted together on the back of the diver, each with its own regulator[REF]) are unquestionably a redundant gas supply configuration, as there is no possibility of flow from one to the other during the dive.