If you're using several motors at once, it's necessary to distribute the mechanical load evenly among them to ensure proper functioning. When you don't balance the load during motor control, they can end up fighting each other and even begin oscillating. When dealing with an imbalanced load, their temperature could spike, or motors may trip due to hitting their maximum torque. You require a solid mechanical connection to spread the load among the motors. So, the deal is that the motors must either work with the same gearbox or be connected through a shaft.
This is a companion discussion topic for the original entry at https://www.solisplc.com/tutorials/balanced-load-distribution-in-multi-motor-systems-using-siemens-plc