But the two motors are not running identically . One of them is faster.
Yes two motors will always run at different speeds unless you take measures to regulate them.
Is this due to insufficient power.
Not quite. It is due ( amongst other things ) to unequal voltages applied to each motor due to your motor drivers not being electrically identical.
But even if you adjust the motor's voltage to be the same two motors can never run at the same speed.
They need to be controlled, that is you need to monitor the number of turns and adjust the voltage so that the two run together.