This YouTube recording has actual video of Nina and Frederik (not just an album cover) from the 1958 movie Verdens rigeste pige. They're billed as "Lisa and Jacque" - I don't quite figure why, but I'm guessing it's a movie about fictitious folkies and they're the main characters.
If you speak Danish, you can find a summary of the plot here (click). The English title of the film is The Richest Girl in the World described on one Website as "Farcical musical comedy, describing hectic courtship between poor little rich girl and a calypso singer." Whatever the case, it's a good example of their singing.
-Joe-