The dual battery connectors allow you to link two batteries together, so when one battery dies, the second kicks in, meaning you get double the range without the need to change over the batteries.
With both batteries linked together via the electric wheelchair dual battery connector, you can charge them at the same time via the controller.
*DISCLAIMER* It is recommended that batteries are at equal level of charge before linking both batteries together, so we suggest draining batteries, ahead of linking them together, and then Charing as one via the joystick once they are linked.