Are you using logic gates to combine the signals, or trying to combine the pins?

If you are trying to combine the pins you will need diodes oriented the right way, PLUS a pullup resistor at the output (or a pull-down).
I suggest active LOW, diodes with cathode to pins and anodes joined, PLUS a pull-up R of a suitable value. 1kohm could be a good starting point.

If you are using logic AND gates, you should also use active LOW and a 2-input gate.

Cheers
Roger

Hi Roger
I am trying to connect the motor output pins together. I have diodes on them already and they set at active low. even though you can disable them in Mach there is still some sort of back-feed  in the system and the servo will not respond to any command, separately they work like a charm.
My cure yet to be wired in but the parts purchased I am going to use one of these. https://www.jaycar.com.au/4pdt-din-rail-mount-relay-cradle/p/SY4004 with one of these https://www.jaycar.com.au/12vdc-4pdt-relay-5a-240vac-30vdc-contacts/p/SY4009 that plugs directly into the cradle and is all screw terminals for ease of fitting up. Using screen 4 I will simply create another button with an "M" command to switch spindle to axis and vise versa. Should work a treat! Famous last words I know. LOL.

I have done something similar (and it works fine).

My CNC is a two-spindle unit: one vertical which I use as a mill and one horizontal, which I use as a lathe. I select which spindle to use with one of two macros. One macro (M90) enables a relay for the mill spindle, the other (M91) enables the lathe spindle. Since I run the mill under Mach3Mill and the lathe under the Mach3Lathe programs, I have slightly different axis Step/Dir pins and different Index pins (different XML files). The first thing either of these macros do is to disable the output pins used by the other macro.

In the external physical logic circuit there is also some lockout circuitry which prevents the system from ever trying to energise both spindles at once. Since the spindle PS is DC, there is also lockout circuitry on the Fwd/Rev relays to prevent them from ever being simultaneously activated.

Yes, a little bit of paranoia - and a lot more peace of mind.

Cheers
Roger