That doesn't sound like a terribly good idea; Calibre's really intended for use in storing all your formats of the same book in a single library entry.
You can easily see what books you have in a given format using the "Format" filter in the tag browser (which is just a search string of the form ' formats:"=EPUB" ', for example) and you could, if you then wished, create a virtual library for each format. I really wouldn't recommend creating separate physical libraries, though, because it would significantly increase your workload if you were, for example, to edit the metadata for a book. You'd have to edit it separately in each library.
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