Hi,
a CNC system is a realtime control system. That is to say that the computer/controller must be able to respond instantly, or as near as instantly, a micro second or so, to an input and be able
to maintain output pulse streams to within a fraction of a microsecond accuracy.

If you had two instances of Mach running would would also require two instances of the parallel port driver running, leaving aside the fact that as written the driver can only be a single instance,
it would require Mach share the CPU, half each. Realtime control goes down the toilet, realtime means that it stays focused on one job and NEVER looks away.

Craig