The other day I wanted to drag a new song I’d purchased at iTunes into one of my playlists. However, there seemed to be no easy way to have two iTunes windows open to do this. It may be easier when macOS Catalina debuts this fall and breaks up the iTunes app into separate Apple Music, Apple Podcasts, and Apple TV Plus apps, but for now, it’s a little convoluted (at least for an Apple product), but you can do it. With iTunes launched:
Go to your Music Library by clicking Library at the top middle of the iTunes screen.
Move your cursor to the iTunes Sidebar on the left side of the app. If the sidebar is hidden, use the keyboard shortcut Option + Command + S to make it appear.
Choose an iTunes playlist.
Right-click or Control-click on the iTunes playlist and select Open in New Window.
Now you can add items to the playlist, delete ‘em, or perform tasks as needed.