(I just thought of this and confirmed it works)
Also, you can have a song playing in Library 1, switch to Library 2, right click a song > Queue Next, and it adds as the next track in Now Playing. Very nice!
The downside is, no matter which Queue option you choose (Queue Next, Queue Last, Queue Album Next, Queue Album Last) after the queue Upcoming Tracks resumes tracks from that first library.
If you wanted to add something, you could include a "Queue From Here" to the Play More List.
This would facilitate a smooth transition from one library into a specific spot of another library. Then, at the end of your queue, Upcoming Tracks would continue playing on from that second library.
It could also be useful for transitioning between playlists.