I purchased a USB Breakout board, motors and drivers, paid for CNC USB Controller Software and I CAN NOT "Slave" an axis, but there is an option to do that. The moment I connect USB Breakout Board "Sl

I would not recommend slaving an axis using the method of taking an additional axis from the USB interface. Instead, wire both drivers to a single axis on the USB interface. This will insure perfect signal output to both drivers. If you requre the motors to spin in opposite directions, simply wire one of the motors to the driver reversing the coil A and B connections, or use a hex inverter (https://www.buildyourcnc.com/tutorials/Tutorial-Two-Motors-Spinning-Oppositely-On-Single-Axis).