Plug in one battery, and both will charge!
The Ranger comes with the same charge port as the stock battery which means it can be charged by your stock charger. When the batteries are connected on the bike, plugging one charger in the stock battery, while on, will charge the whole system together.
Keep in mind, double the capacity means double the charge time!
You can also plug in two chargers at the same time to charge faster. You can even use both the stock charger AND fast charger at the same time. Even though the chargers are at different rates, they will charge the whole system at the sum of both charging rates. When the batteries are connected just think of them as one big battery with two charge ports.