Driving two stepper motors using one stepper motor driver
If we call the motor leads A1, A2, B1, B2 on each motor and control, you could tie the two motor A1's to A1 on the control, and motor B1's to B1 on control.  Then wire a DPDT relay as below.  (Coil not shown.)  Note, DPDT relays are about $4 on ebay, with labels like “DR21A01 DC 5V DPDT Relay Module Polarity reversal switch Board for Arduino UNO”. 







simulate this circuit &ndash; Schematic created using CircuitLab

Since you have equipment in place and working, it might not be worthwhile to change your setup.  However, even without adding a relay, you probably can power both motor controls and motors with a single power supply, ie could use one instead of two power supplies.  The current draw when a stepper motor is idle is a small fraction (eg 10-20%) of the high current drawn when the motor steps.  If you don't have power attached to a stepper motor, there is only its cogging resistance to keep it from turning; it does not take a large torque to turn an unpowered stepper motor.  If that's a concern, the DPDT relay approach might be unsuitable.