Official comment
If you have two separate databases which are currently on two computers or one internal and one external, here's what I suggest...
Decide which will be the "master" database.
Open the other database and export all the playlists and songs to a USB drive.
Open the master database, right-click on your playlists folder on the export device and select "Import to Collection." You'll be asked if you want to copy the music as well, choose yes and select a destination for it.
That entire set of songs and playlists will be imported, the music copied over (you can move it later, don't worry), and then the two databases will effectively be one.
Wipe the export USB drive, then move the music back to it if you want to have the music stored externally, and perform a relocation.
Export your playlists to that drive - rekordbox will identify that it's also the host source for the songs in the collection and it doesn't need to export anything but the waveform data and export database (to be read in CDJs / XDJs).
Let us know if you need any other info or have any questions before you do this. ;)