In fact, UCCNC takes the problem away because it does support the two-probe setup out of the box. The issue was with Mach3 which, according to the documentation, does not. I was happy that I could find an electrical "OR" solution but it was going to be messy with my existing hardware. That's when I realised that the Mach3 and macros that came with the machine clearly did work with two probes and I found the "setdatainput" command in the macros supplied. It is virtually unmentioned anywhere (according to Google) although I found a reference that says it is only in .066. The current question is about whether or not that command is in the French version. Not clear why it would not be, but details of Mach3 development are lost in the mists of time...