To anyone still having this issue or finding this thread when searching online: "How to clone a VM between hosts in Xen Orchestra" or similar...

The key piece of information missing from all these answers is that you must first connect both hosts (XCP-ng) servers to the same Xen Orchestra instance before you can clone (copy (export/import)) a VM from one host to another while using the Xen Orchestra GUI. Note that as mentioned in the first reply that this is only one solution, given both hosts can see each other over the network. You can also manually export and import the VM as an image file to your local PC's storage if needed.

To do this, first navigate to "Settings->Servers" and configure/connect the destination host alongside the existing host so you end up with two hosts connected to the same Xen Orchestra. Note that Xen Orchestra can display information and manage multiple hosts if required (does not have to be tied to a single XCP-ng instance).

Then navigate back to "Home->VMs" and you should now see all VMs listed for both hosts in the same list. From here you can select the VM to clone and from the copy dialog menu when you select a "SR" you should now be able to select between both host's disks (Local Storage or Pools) to copy the VM onto. Note that this command will spin up two separate tasks: an export and import task simultaneously on each host to complete the copy (sort of streaming it at the same time). Also don't forget to first shutdown the VM first before you copy it so you do not risk losing/corrupting any live data while it is cloning.

Hope this helps everyone.