Taking up the suggestion of instrumental pedagogue Egon Saßmannshaus, Bertold Hummel wrote some easy duos for two violoncelli. They are to be found in:
Egon Saßmannshaus:
Early Start on the Cello
A violoncello
method for children of 4 or older.
Vol. 2:
Bärenreiter
BA 8997
Andante (1976)
Presto
(1976) 
Version for 2 Violins
Herbstmelodie
(Autumn Melody) (1976)
Musette
(1976)
Version for 2 Violins
March
(1976)
Version for 2 Violins
Siciliano
(1976)
Egon
Saßmannshaus:
Early Start on the Cello
A violoncello
method for children of 4 or older.
Vol. 3
Bärenreiter BA 8998
Hornpipe (1987)
Canon in The Lower Seventh (1987)
Elegie (1987)
Tango (1987)
Burlesque (1987)
In Fifths (1987)
Egon
Saßmannshaus:
Early Start on the Cello
A violoncello
method for children of 4 or older.
Vol. 4
Bärenreiter
BA 8999
Alla
marcia (1989)
In The Tempo of a Tango (1989)
Berceuse
Lullaby for Jacki
See also: Works by Hummel on YouTube