Help needed with CNC shield!

I am making a CNC router for a school project and I have run into an issue. I need hook up 2 motors to the X-axis driver, and also make them spin in the opposite direction. So basically, I need to mirror the output of the X-axis and use that for another motor. The CNC shield I use has space for 4 stepper drivers but I don't know how to control the fourth one named 'A'.

I am using an Arduino UNO with a CNC shield extension, 4 A4988 stepper motor drivers and 4 NEMA 17 stepper motors. I use GRBL on my Arduino and universal G-code sender to control the motors.

I have thought about Soldering two sets of cables together in an opposite and parallel way but some people told me I will short out the circuit doing that.

Some help would be Greatly Appreciated!