What on earth would I want with two shafts?
It’s a very good question!
The dual shaft can be used in a number of ways. One example is to drive 2 applications on either side of the motor. A second (and by far the most common) is to use the rear shaft to mount an encoder, brake or hall sensor array in order to be able to set up a closed loop speed control system.
Depending on your specific requirements these add-ons can then be used without having an impact on the main drive shaft. For example an encoder can be used to turn an open loop stepper motor system into a closed loop system.