Run a stepper from 2 DM542 drive

[A_Camera]

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.

That is only true if he ALWAYS wants to have the stepper energised and always holding. That is not always desirable or even advised. So unless you absolutely certain that he doesn't care about enabling/disabling the driver it is indeed necessary to switch the enable signal also. Regardless of which, there is no harm in switching it, but there can be harm in not switching it.

And the GND is only needed for the main power, as the driver inputs are optoisolated.

GND is the same as STEP - , DIR - and EN -. It is better to connect those to the same - output as the one providing the control signals. Again, it would do no harm and we know too little about his design and implementation.

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.

That's why I said that a small signal relay needs to be used, not a contactor. You talk about a contactor which is designed for high voltage and current.

[m_c]

That is only true if he ALWAYS wants to have the stepper energised and always holding. That is not always desirable or even advised. So unless you absolutely certain that he doesn't care about enabling/disabling the driver it is indeed necessary to switch the enable signal also. Regardless of which, there is no harm in switching it, but there can be harm in not switching it.

If you'd looked at the relevant photo, you would see the Enable on the driver has nothing connected to it, plus there is also the post stating he doesn't want the axis to become disabled to avoid the axis dropping, so connecting the enable is not needed.

GND is the same as STEP - , DIR - and EN -. It is better to connect those to the same - output as the one providing the control signals. Again, it would do no harm and we know too little about his design and implementation.

Not on the existing step source, as again, the photo shows differential outputs.
However I did think about this earlier, as the alternative controller may not have differential outputs, so some thought may be needed in that regard.

That's why I said that a small signal relay needs to be used, not a contactor. You talk about a contactor which is designed for high voltage and current.

All relays, apart from specialist relays have minimum switching currents. It is not something that only applies to contactors.
Good relay manufacturer's will publish the relevant detail in the datasheet.