When trying to use two motors on a single axis, there is a multitude of methods to get them to work together.
First: There should always be a slave option in the CNC control software(mach 3/ planet-cnc/emc2/etc.), this will be the easiest way to make the dual motor configuration work. However some adjusting might be necessary due to the orientation of the motor when mounting it on the opposite side of the CNC machine.(Mach3/config/slaveaxis, planet-cnc/file/settings/axes).
Secondly: Dealing with our interface boards(maybe third-party as well), you can have two drivers going to the same axis on the interface board. Which then will have one motor per driver, this will use the same motion and control from the (ex.) x-axis to driver two motors. However some adjusting might be necessary due to the orientation of the motor when mounting it on the opposite side of the CNC machine.
Adjusting of the driver or motor wires, can be done separate from the control software with the use of a hex inverter, that can be used and to switch the signal (ex. takes a low signal and brings it high, and takes a high signal and brings it low) of one of the motors, to run the same as the other motor.
There is also another method of inverting the orientation of the motors movement without the use of a hex inverter. This method you will have to wire the coil's of the motors oppositely of what is recommended for one of the motors. Example, you will wire our Nema 24 as follows(recommended): A+ - red/blue, A- - yellow/black, B+ - white/brown, B- - green/orange. However to run another motor with with it you will have to switch the A/B connections to: A+ - white/brown, A- - green/orange, B+ - red/blue, B- - yellow/black.
These method's are usually needed/used when trying to control two motors and setting it up without the help of the CNC control software, and also due to the mounting orientation of the second motor, the inverting the direction of motion will be necessary so they work together instead of working against each other.
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From the BOB how can I reverse one of my steppers - i'm running 2 on my Y Axis