You can point both iTunes libraries at the same music location – go to “Edit” -> “Preferences” -> “Advanced” -> “Change”in one of the libraries
Set this to the location of the music in the other library (On windows, My Music -> iTunes -> iTunes Media) and then the same music will be in both libraries.
BUT if you add/delete tracks in one library you will then have to go to “Add File To Library” to add that song to the second library as it won’t be added automatically. And if you delete a track in one library you then have to delete it in the other as well.
🙂