This is the first time that we have ever heard of a problem like this so this is not a common occurrence for other Razor Ground Force Drifter owners. The controller does not provide regenerative braking so when the go-kart is decelerating none of the Voltage generated by the motor is fed back into the battery pack.
The battery pack is wired in series and both batteries are discharged and charged equally so we can not make heads or tails as to why the inside battery would be the one that keeps failing. We are curious what testing method you are using to determine that the inside battery is bad? Also, what is the at rest Voltage of the inside battery and outside battery?
Both of the batteries in the battery pack need to be the same brand, same model, same age, same Voltage, and have the same usage history, otherwise their Voltage levels will become progressively unbalanced during discharging and recharging which will cause one of the batteries to be overcharged and the other battery to be undercharged. Overcharging and undercharging of individual batteries in a battery pack causes them to have a short lifespan and fail much faster than normal.
To solve this problem both of the batteries in the battery pack need to be replaced at the same time with new batteries that are the same brand and model.