Dual Battery Discharge Converter 40A 24V-72V XT60 Plug Compatible with E-Bike Single Motor, Battery Parallel Module Switch Balancer for 750W/1000W Motor

I don't have the same bike but similar. you just need to match voltages.https://www.amazon.com/RANSYRI-Universal-Battery-Waterproof-200-1500W/dp/B0F1CTBXW9/This would work, you would probably want a battery balancer if you want to hook them up at the same time.https://www.amazon.com/Etersec-Discharge-Converter-Compatible-Parallel/dp/B0FRF2D3SY/If this bike uses XT60 connectors, its super easy. No splicing or soldering required.