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!
Plz share the link of the particular CNC shield that you're using.

Hi
Could you please share the SKU of the shield? I can not find it in our store.
Looking forward to your reply.
