Setting up a 4th Axis (new to CNC)

Pardon my newness, but I have run into an immediate snag. Sorry if there is a post on this already, as I searched but didn't find. My machine actually uses 2 stepper motors to run the Y axis--one on each side of the gantry. Each motor is run to its own driver control. I set up the usual X, Y, and Z axes per setup tutorial videos and also added an A axis (as my second Y). The A axis motor needs to mimic the Y axis motor. Can I set this up using Mach 3, or do I have to account for the 4th axis in g-code?

The good news is that the software setup went fine, the parallel port is getting healthy readings, my wiring is fine, and I am able to jog on all 4 axes. The bad news is that when I try to run a test program, only one side of my table's y axis moves (the side I designated Y--the A axis side gets no code info, so it doesn't move). Hopefully this is an easy fix as I do not want to tediously fix g-code if I can avoid doing so. Thanks for the help.