It is best to use alone, electric vehicles can not just add batteries, adding batteries means pressurizing the battery, originally 48V plus a section of voltage changes the controller can not bear, the voltage is too high the converter can not bear easy to burn out.
As long as the 48V battery has no effect on the motor. The main thing is storage power.
In this way, you still use 2 fast 12AH batteries, change a good quality controller about 60v and then change the circuit and converter, or find a battery with the same voltage and low capacity to use the 2 fast strings for the purpose of voltage reduction, or use a switch in parallel, a battery is used up to convert another.