Taking up the suggestion of instrumental pedagogue Egon Saßmannshaus, Bertold Hummel wrote some easy duos for two violas. They are to be found in:
Egon Saßmannshaus:
Früher Anfang auf der Bratsche (Early
Start on the Viola)
A viola method for children of 4 or older.
Vol.
3: Frohes Duospiel (Merry duo playing)
Bärenreiter BA 9683
Presto
(1976) 
Version
for 2 Violins
Andante
(1976)
Musette
(1976) Version
for 2 Violins
Herbstmelodie
(Autumn Melody) (1976)
Siciliano
(1976)
Marcia
(1976)
Version for 2 Violins
Egon
Saßmannshaus:
Früher Anfang auf der Bratsche (Early
Start on the Viola)
A viola method for children of 4 or older.
Vol.
4
Bärenreiter BA 9684
Alla marcia
(1989) Version
for 2 Violins
Tango
(1989) Version
for 2 Violins