Yes, you can use 2 motors in the same axis output, however you will still need a driver for that motor! Also depending on the orientation on which you mount the motor you might have to invert the direction of the motor, and that will be simple by swapping the A+,A-, to the B+,B- locations and vice versa, from the driver to the motor wiring.
Also you can run a slave motor using another axis on the board, and setting it up in the Planet-CNC settings.
Planet-CNC/File/Settings/Axes, here you will enter 3 in the Number of Axes location, and then change the Function of the Axis 4 to Slave 1. There you will have the 4th axis or A-axis be a slave for the x-axis.
Slave 1 - X-Axis
Slave 2 - Y-Axis
Slave 3 - A-Axis
Slave 4 - B-Axis
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Can I run two stepper motors off the same axis output on the USB controller?