Be careful. First, take a multimeter and measure the voltage between the grounds. For example, you put step to 5v from one psu, and driver will have it referenced to the other psu ground.
So measure between from your controller 5v to the ground of the other psu, and see if there are 5v or not, this with grounds disconnected.
So measure between from your controller 5v to the ground of the other psu, and see if there are 5v or not, this with grounds disconnected.