For the record, according to the ntl.matrix website, Frankie Laine's version reached #17 in 1955, Les & Mary's version reached #7. It was very common for 2 or more versions of the same song to chase each other up the charts.
About harmonies, note that by the time this song was recorded the use of multi-track recording had rather recently been perfected - by none other than Les Paul, who was something of a technical genius. So, Mary Ford was able to harmonize with herself, something that was not possible just a few years earlier.