Hi,
all development on Mach3 ceased eight-nine years ago. Notwithstanding that it still works its obsolete, consider Mach4. I've been using it for eight years and its light years ahead.
One of the areas where Mach4 excels is slaved motors.
In Mach3 you had to re-envision a rotary axis as a linear axis and slave it to another, usually a linear axis.
In Mach4 you have one master motor per axis and up to four slave motors to each master. The master/slave relationship can be broken and/or re-established at will by programming
and so you can build very sophisticated homing and squaring routines. Some of the better known motion control boards like the Ethernet SmoothStepper handle squaring a two motor gantry
autonomously.
One principle that Mach3 relies on is that both motors, master and slave, must be identical, that is to say steps/unit, max velocity, accel etc.
Mach4 can have two different motors of different tuning....although I think while it's possible to do so, how well it works in practice is another matter.
Craig