Hi Albertaclipper,
I run a slave axis and I have set X & A in Mach3 to mirror each other (much simpler for my tiny brain to comprehend) and have just swapped the wires to the motor B+ & B-.
I had concerns recently about one motor stalling and the other continuing and doing unspeakable damage to my gantry. I have updated my drivers to ones that have a stall detect on them - which is is wired back into the E-stop. My motors have drive shafts on both sides. The outsides are driving my axis and I have connected the insides together with a shaft, the idea behind that is that if one starts to labour a bit the other one will help keep it running, kinda like hooking up two locomotives in tandem. Maybe a bit of overkill, but it works.
Cheers
Rod