In the latest Zotero beta, we've added automatic merging of adjacent in-text citations by the word processor plugins. It works in Word, LibreOffice, and Google Docs.

To merge citations, make sure they're immediately adjacent, with no spaces between them, and then you can just press Refresh to merge them into a single citation.

For Google Docs, you'll need to use either the Zotero Connector beta for Firefox or the Zotero Connector for Safari included with the beta app.

For LibreOffice, you'll need the updated word processor plugin included with the beta app, so if your system doesn't allow Zotero to automatically update the plugin (e.g., due to security restrictions), you'll need to manually reinstall it.

(And just as a reminder, if all you want to do is create a multi-item citation while you're writing, or add another item to an existing citation, you can do that right from the citation dialog — you don't need this feature.)