However, when I create a virtual copy on the first Mac, I realize that the corresponding xmp file gets updated.
As far as I know, xmp does not contain information about virtual copies. So this method of attempting to synchronize the two catalogs will not work.
Better you should use File->Export as Catalog on one computer and then File->Import from Another Catalog on the other computer.
Thank you very much for hint. I got the point. Indeed, the xmp file do not seem to contain any information about virtual copies. Do I need to consider something if the image folder already exists in the Lightroom catalogue on the second Mac?
From File->Import from another catalogue I understand that this will work perfectly if the folder to be added has not been previously added to the Lightroom catalogue on the second Mac. This situation only occurs during the first synchronization.
The next time, the folder is already imported on the second Mac and I only want to "transfer" the metadata and virtual copies from the local Lightroom catalogue of the first Mac to the local Lightroom catalogue of the second Mac, where the images are stored on a network device that is accessible from both Macs.
Will I encounter problems if the folder has been previously loaded? Or do you recommend to delete this folder from the second Lightroom catalogue (without deleting the files on the drive behind)?
It has nothing to do with the presence or absence of a specific folder on the 2nd computer.
File->Export as Catalog and then File->Import from Another Catalog works in any situation, not just the one you stated. It will do what you want, that is:
I only want to "transfer" the metadata and virtual copies from the local Lightroom catalogue of the first Mac to the local Lightroom catalogue of the second Mac
I do not doubt that File Export / File Import will work to transfer the virtual copies.
My question was related to the fact whether the imported folder should not previously exist on the second Mac.
Consider the following situation:
time = 0:
first Mac: folder A is in a first status
second Mac: folder A is in a first status
time = 1:
first Mac: folder A is in a second status, export folder A in second status
second Mac: folder A is in a first status, import folder A in second status
My question is: What happens with these two folder A on the second Mac? Does folder A in the Lightroom catalogue of the second Mac gets overwritten? Do I receive an error message because folder A is already present in the Lightroom catalogue?
I understand your answer that everything will be fine on the second Mac if there is no folder A at all at the second Mac while I import the new folder A. Under this assumption, I can import folder A and it will show the second status. Clearly.
My question is only related to the fact that there is already a folder A on the second Mac, with the same images, but with different metadata and/or virtual copies.
Thank you very much for your support. Updating works fine!