On the left menu, select Playlists. Use one of the 3 icons below to edit, delete or combine playlists.
The Combine icon allows you to combine playlists in various ways. Select the operation and which playlists you'd like to combine, and then click on Combine Playlists. The join playlists option will just add the two playlists together, so if there is a recording that is in both of the playlists, it will appear twice in the joined playlist. The intersect option will take the two playlists selected and create a new playlist that contains only the recordings that are in both of the intersected playlists. Removing playlist 2 from one will create a new playlist that has only the unique recordings from playlist 1.