AKA "Cada Vez," Le Sable et Mer."  Although I've heard the musical term "quad-libet" applied to this, I'm not sure what this means.  All I know is that (in the style of 'Memories Are Made Of This', by Terry Gilkyson) you can write several melodies to the same chord pattern, and sometimes sing them at the same time.  This was written for a boy/girl duet, and did pretty well for Steve Lawrence & Eydie Gorme and (in Canada) Jacques Boulanger & Ginette Reno.  I've always liked this form of music, as you can tell two stories at the same time, with two different points of view.  (It's an obvious venue for boy/girl duets or statement/comment, which allows the writer to look at situations from differing points of view.)