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snoopyeab You can open two libraries from two virtual machines using something like Virtual Box
You could output a book list for the two libraries via the calibredb list command, and use a file compare command to see the differences. Also there are gui tools to compare differences between text files - WinMerge is said to be quite good and its free. I'd use Beyond Compare, but its not free and it's probably overkill for what you want to do.
BR