Are you talking about personal libraries or group libraries?
For personal libraries, accounts can't be merged, so if you'd like to combine two libraries, you'd have to export from one (to Zotero RDF with files/notes) and import into the other. You'd generally want to keep the one that you've used with any word processor documents, since citation links would be lost via export/import.
If you're talking about libraries that appear within the same Zotero app, you can drag collections and items between libraries.
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For personal libraries, accounts can't be merged, so if you'd like to combine two libraries, you'd have to export from one (to Zotero RDF with files/notes) and import into the other. You'd generally want to keep the one that you've used with any word processor documents, since citation links would be lost via export/import.
If you're talking about libraries that appear within the same Zotero app, you can drag collections and items between libraries.