Run a stepper from 2 DM542 drive

[m_c]

I've not really been following this thread, but the simplest option is a 4PDT relay as has been suggested, but you only need to switch the Dir+/-, and Step +/- signals.
Driver is permanently enabled, so you don't have to worry about it losing power and the Z dropping.
And the GND is only needed for the main power, as the driver inputs are optoisolated.

The only issue with using a relay is with switching such a small current, the contacts will likely oxidise with use due to lack of current to wet/clean the contacts, eventually leading to lack of continuity,
For a circuit only switching occasionally, it'll likely be a long time before failure is likely to be an issue, but it's something to bear in mind.