Using the Robocopy across the Virtual Machines

Solution 1:

You're asking multiple Things, however I try to be specific as possible

Do I need to keep both VM on the Same Domain for robocopy to work, or I can use it over cross-domain

• No, They don't need to be, as you can use a network Drive to map the paths and copy it from there.

I was told that if a VM is on a Domain then all Administrators who have AD Entries can access the VM

• Yes, Any Domain Administrators with the Specific rights can access any Device that is allowing it.

If the VM is Not on a Domain, that is, Kept in WORKGROUP, then access is automatically denied.

• Yes because only the Local Credentials will work, remind if it was installed via an Automatic Deployment, it may that these Administrators Deployed LAPS or already set a User and Password - This does not meet in case you used a self created installation device.
• Remind that in this case, this Device may or will be seen as a non-legit and illegal device which can bring you into trouble. Some Companies will escort you directly into the Outside. This sentence is meant in case you are not allowed to do so

And Also as additional information: Your Question may also already been answered with ServerFault Question 203959