[BrianG]

Both plugs are only needed if you plan on using over 5A due to the current-carrying capabilities of the servo wires and connectors. If you are just using regular servos, or just one servo, then you really only need one plug since current draw will likely be less than 5A.

There are other ways you can wire it up to use both plugs, even with all ports being used. But it involves modifying the ESC and/or steering servo wires.