Well, I might be confused about this, but isn't having a second string to your bow another way of saying "two-timing"? and wasn't Johnny two-timing Frankie? Seems like it might be kind of a back-door allusion to the root of the whole sad situation.

My favorite line to "Frankie & Johnny" (from the Burl Ives version I grew up with) has always been: "It was not murder in the second degree; it was not murder in the third. Frankie just went and dropped her man like a hunter drops a bird..."