Couple Stepper to AC motor
If one motor is completely disconnected when the other is operating, it is possible to drive the load with one motor turning the other. If the idle motor is disconnected, there should be no problem with the motor acting as a generator. If one motor is a permanent-magnet type, it will present some pulsed reluctance torque as a load to the other motor. That will result in some torsional vibration, but it would not necessarily be harmful to either motor or the load. Both motors must be mechanically capable of the maximum speed. If one motor is a permanent-magnet type, the voltage generated when it is idle must not exceed the motor's insulation rating. The motor that is in the middle must be mechanically capable of transmitting the torque of the motor on the end. The motor that is driving must be capable of driving the inertia of the idle motor. There may be alignment or overhung load issues if one motor shaft is much smaller than the other.