There are two things to consider when using two stepping motors on a single axis. Since you want both motors to spin at the same rate and possibly in the same direction, you need to connect both motor drivers to the same breakout board pins. For example, if you want two motors on the x-axis and the x-axis pins on the breakout board are #2 for step and #3 for direction, the #2 will connect to the step (cp) terminal on each driver and the #3 will connect to the dir (cw) terminal on each driver.
However, if the motors will be opposing (one motor will need to spin in the other direction), you will need to consider either flipping the rack on one side, or adding a circuit to invert the direction (#3) signal going to that driver.
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I'm planning a rack and pinion style gantry, and need clarification on how to slave a second stepper to the Y (or X). Can you direct me to that information for your products?