There's only one music/audio track, so you need to get a bit creative....
you could:
1 - overlay two audio files by dragging one over the other so they both play
together.... a 5 minute narration file can easily be dragged to the left in
the timeline over a 4 minute music piece so they would play together.
2 - save the movie with the narration or music, and then import it as a new
project and add the other one.