I've had a bit of a play ..... I can use the same input but one probe requires a HI signal and the other a LO, so as it stands now, I still have to change the configuration in Mach 3 each time I change probes.
There are two different situations here. The first is where you have touch probes with different outputs but only want to use one at a time. The other is where you actually do have two different probes running in parallel - and yes, such situations do exist with value. If I could figure out how to attach an image to this post I would include a circuit diagram for the two cases.
Oh well, in words then. I don't run the touch probe straight into the Mach3 pins. I use a pull-up (or pull-down, as the case may be) resistor, then a couple of 74HC14 buffer gates, with a switch to select polarity. Some RC noise filtering is included with the gates, and then it is all fed through an opto-isolator into the Mach3 pin. The filtering AND the opto serve as very good blocks against noise.
Cheers
Roger