Hi, Since you/your Arduino has control of the motor, it would be best to measure the cell voltage(s) with Arduino, and make your own decisions, discharge warnings, shutdowns etc.,.
If you use a 2:1 voltage divider like two 100K resistors in series, you could measure the 7.2V point, and you could measure the 3.7V point directly, on two separate Analog inputs.
Subtract bottom Cell from total to get top cell??