It looks like this is a "home-made" mix of the two versions rather than a "rare LP version" ...
Interesting to see that the first version to be recorded was by Marvin Gaye, even though the first to be released, by many years, was the Diana Ross version.
From DFTMC ....
"Dark Side Of The World (Nickolas Ashford-Valerie Simpson) published Jobete 17-Dec-69
alt title: Don't Leave Me On The Dark Side Of The World
Marvin Gaye; recorded Hitsville-GW, completed 26-Mar-69 ; produced by Nickolas Ashford, Valerie Simpson
Mar-86; LP (S): Motown 6172 Motown Remembers Marvin Gaye [with overdubs added 1986]
28-Sep-99; CD (M): Motown 153 868 2 Love Starved Heart - Expanded Edition
26-May-09; CD: Motown UICY-94127 Motown Remembers Marvin Gaye [with overdubs added 1986] [Japan]
Diana Ross; recorded Hitsville-GW / L.A., completed 21-Jan-70 ; produced by Nickolas Ashford, Valerie Simpson ; arranged by Paul Riser
06-Apr-70; 45 (M): Motown M 1165 B [cv]
Jun-70; LP (S): Motown MS711 Diana Ross (retitled Reach Out)
16-Dec-91; CD: Motown MOTD 5135 Ain't No Mountain High Enough
26-Mar-02; CD: Motown 016 818 2 Diana Ross
27-Jun-08; CD (M): Hip-O Select B0011056-02 The Complete Motown Singles Vol. 10 1970"
Very nice anyway.

Roger