You will want to export one library as Zotero RDF and import the .rdf file into the second library. To do this, click “File” → “Export Library…”. The default export format should be “Zotero RDF”, which you should leave selected. You can check the boxes for including files and notes if you would like to transfer those to the other library. Click OK, name the .rdf file, and choose a destination to save it. You can then e-mail this folder to a friend or colleague or to yourself. From the second Zotero library, click the “File” menu again and select “Import…”. Choose the .rdf file in the folder you created and click OK. This should import the items, combining the two libraries.