Ken, being a concert band tuba player since high school, I checked a couple of different sources, including The Boosey & Hawkes website, Hal Leonard (who distributes the Boosey & Hawkes catalogue in the USA), a "public domain" scanned full score and parts, and well, Wiki to try to find further links. It appears there was/is only one part for "Basses"; no separate part for double bass, at least in the original concert band edition. The octave splits are for Eb tubas and Bb tubas and/or where the player wants to choose to "break back" the runs.
http://imslp.org/wiki/Second_Suite_f...(Holst,_Gustav)