I don't need to mess with switching homes/vfd signals etc - the wonderful CSMIO has so much I/O that i can connect both systems at once and use the different Mach profiles to do the control.

It has two 0-10v outputs for the spindle motors, most of the functions can be duplicated, just the Z-home switch, 0-10v and the Run Fwd need their own outputs.
Nice.
Also, as we don't need to switch control lines now, I think the spindle VFD required can be controlled via its own output using the HV output in CSMIO and a small contactor/relay combo.
Even nicer

So startup will be just a matter of choosing the right shortcut for Mach to launch and let the machine get on with the settings while i fit or remove the high-speed spindle.
One little hiccup to my smoothness here is that the spindle needs to be parked in a position low enough to enable the aux spindle to be fitted before OR it means starting one profile then lowering the spindle then closing and starting the aux profile - not as smooth;)