It is possible to only use 1 current sensor, the BMS will also work perfectly fine. Using one sensor will use less power on the Begin Board. The drawback is that you cannot measure incoming and outgoing currents independently, but only see the current going in or out the battery pack. Connect the current sensor to Sensor 1 on the Begin Board and make sure to see a positive current in the App next to the battery icon when charging and a negative current when discharging.