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If you have let's say two epubs of different editions of the same book (e.g. one illustrated, the other not), you could convert one to AZW3 and indicate within the metadata (e.g. in a custom long text column) that the AZW3 is the non-illustrated edition. But this approach unlikely to work satisfactorily if they are PDFs or some other 'difficult' to convert format.
If it's only a few books the .7Z archive method is probably the best, we suggest .7Z because calibre won't do anything with them, whereas it can process .RAR and .ZIP archives. If it's a lot of books you might want to consider keeping them in a 'supplementary' folder and putting a link in an Identifier value, or in a long text custom column.
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