Yes, you can. You would need a board to invert the direction signal but its very straightforward. I did it on 2 separate boards but the same principles apply on 1 board. You could just disconnect the corresponding direction and step pulse pins so that they do not plug into the arduino. Then you could jump the two step pulse pins and run the direction pin of the axis you are copying through a small board that inverts the signal(0V–>5V & 5V–>0V). The pinout can be found here:
https://github.com/grbl/grbl/wiki/Connecting-Grbl
And my original post about using 2 motors with one signal is here:
2 g-shields from 1 arduino?
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