According to the list of works for viola on wikipedia, there are a number of compositions for viola and piano by Zitterbart... He wrote a total of 13 Viola Sonatas, and several smaller pieces for the same instrumentation. Many of them are available here in manuscript form here:
https://digital.library.pitt.edu/collection/fidelis-zitterbart-manuscripts. The American Viola Society also published his Sonata number 2 in G-minor (beautifully edited by David M. Bynog in 2015) and his Three Pieces. These are available at the American Viola Project:
http://www.americanviolasociety.org