Can you afford to lose the playlists from one of the libraries? I can't think of a way to keep both library's playlists, sorry. 😱
Use the iTunes Preference pane to move your internal drive's library to the external drive. This is done through the Advanced panel where you Change the library location. Then, simply drag the second library into the first and they'll add albeit without their playcounts/playlist info.
Hopefully someone else will come up with a better way though. 🙂