Great question. The reason you find very little info, is that you can’t just wire two motors to the same driver. Not a wise idea.
Instead, you “Mirror” the second axis. You can do this two ways:
- The Bodge. You can physically cut some traces on the PC board to disconnect the A-axis ‘Step and Direction’ signals from the input connector, then run the Step and Direction signals from, say, the X-axis signals into the D connector, and route them over to the A-Axis also. Then you have one chip driving one motor, but both chips receiving the same input signals.
- The right way. In the LinuxCNC STEPCONFIG utility, you can specify what signals are output to what pins. Just make the A-Axis into another X-Axis highlighted in the image below. You can mirror any axis in this manner.

I would imagine there would be similar configuration settings for all CNC software.
Let me know how it works out!