If you copy the meta...backup.json to the 'other library' and do a library maintenance->restore database then I think the only file that will change is the metadata.db, and a pre restore database will be created. The files in the book folders wont change - ergo they wont need to go the cloud
But its is NOT a procedure I've tested, it may eat your library
BTW dont synch to cloud while calibre is running see 3rd para ==>>
Don't put your calibre library on a network*
BR
* Nor your daughter on the stage, Mrs. Worthington (ack Noel Coward)