if they have separate histories -- i.e. you started the current one from scratch rather than by deleting items from the old one -- do the following.
1. Sync the current database
2. Back it up
3. Restore the old backup: https://www.zotero.org/support/zotero_data#restoring_your_zotero_library_from_a_backup1
4. Sync again
And you're done.
1. Sync the current database
2. Back it up
3. Restore the old backup: https://www.zotero.org/support/zotero_data#restoring_your_zotero_library_from_a_backup1
4. Sync again
And you're done.