Hi all,
I see it's more than a year ago, but did you ever discover how to resolve this? If not, I can shed a bit of light...
ASDA-A2 series drives have built-in gantry function. You can select it through a parameter in the configuration software ASDASOFT. You should also cross-wire both master and slave servos. The OA/OB/OZ from master goes to the encoder inputs on slave, and the OA/OB/OZ on the slave goes to encoder inputs on the master. You then send the same pulse train to both drives into the high-speed pulse inputs H/pulse, etc.That way the slave sees when the master took a step and the master sees when the slave took a step. The control algorithms are performed by the onboard functions specifically for gantry mode on the A2 drives. You don't need to generate slave pulses from Mach3.
If you haven't already sorted this out, let me know as I have a diagram somewhere.