Merging/Syncing 2 iTunes Libraries w/o creating duplicates

You either need to be incredibly selective about what you add, or add everything and dedupe after...

Apple's official advice on duplicates is here... HT2905: How to find and remove duplicate items in your iTunes library. It is a manual process and the article fails to explain some of the potential pitfalls such as lost ratings and playlist membership.

Use Shift > View > Show Exact Duplicate Items to display duplicates as this is normally a more useful selection. You need to manually select all but one of each group to remove. Sorting the list by Date Added may make it easier to select the appropriate tracks, however this works best when performed immediately after the dupes have been created. If you have multiple entries in iTunes connected to the same file on the hard drive then don't send to the recycle bin.

Use my DeDuper script if you're not sure, don't want to do it by hand, or want to preserve ratings, play counts and playlist membership. See thisthread for background, this post for detailed instructions, and please take note of the warning

to backup your library before deduping.

(If you don't see the menu bar press ALT to show it temporarily or CTRL+B to keep it displayed.)

The most recent version of the script can tidy dead links as long as there is at least one live duplicate to merge stats and playlist membership to and should cope sensibly when the same file has been added via multiple paths.

tt2

To an extent ... that will result in one set of folders that contains the media from both libraries, but neither iTunes database will reflect the merged content. If you can use a synchronization method that avoids mass duplication at the file/folder level (which will depend on both libraries having the same organization and naming conventions) then it may be better to create a new iTunes library (SHIFT-start iTunes and select this option), and then import the content of the merged media collection. If you do this, however, you'll lose all previous information related to playlists, playcounts, ratings, etc. and may have to reload artwork that was previously retrieved from the iTunes Store (Get Artwork function).

You're 100% correct that "there is no easy, straightforward way to do this".

"It's very disappointing that there Apple doesn't provide support for such functions."

Agreed, though it has to be assumed that Apple is limited in its ability to support capabilities that would be perceived as making it easier for media to be copied between users. In establishing the iTunes Store Apple will have entered into all sorts of complex agreements with the content providers (record labels, movie & TV studios, ...) - and we see the consequences of this in some of iTunes' inherent limitations. Even though additional capabilities like Home Sharing and Family Sharing have started to blur some of the edges, iTunes is still essentially based around a "one user, one computer, one iTunes library" paradigm.

As far as your original issue goes, I did on one occasion have a vaguely similar solution (inasmuch as I had content on one library that I want to add to a second, where the two also had some overlapping content). The approach that I used was to (painstakingly) compare the content of the two libraries, using this to create a playlist with the "secondary" library that included all the content missing from the "primary" one. I selected all the content of the playlist (not the playlist itself) and dragged-and-dropped this to a folder on an external drive. I then connected the external drive to the other computer, with the "primary" library, and added the content of that folder. Obviously this is entirely dependent on "manual inspection" to reduce the potential for duplication, but may be viable approach in the absence of any other, more automated methods.

Good choice - SyncToy is also a very effective method for creating and maintaining a backup of your iTunes library, as well as for replicating one "logical" library across two or more distinct systems.