I have a sn754410ne IC (quad half h-bridge). I noticed that whenever driving two motors, one motor is driven faster than the other (with both DC power and a PWM signal from my Arduino UNO). I measured the voltage across each motor and noticed there was a discrepancy of .015V. What can I do to drive the motors at the same speed, aside from sending a smaller PWM signal to the faster motor?
The reality is that your two "identical" motors are only approximately identical; there are always going to be minor manufacturing variations. If you want to run them both at a more accurate approximation of the same speed, you will need to provide feedback (of the motor speeds) to your controller so that it can adjust the drive PWM signals so that the speeds of the two motors match.