There is an early work by Elliot Schwartz, a sonata for violin and bass. Go to his website and contact him, he may be able to get a copy to you.
Duo Concertant by Niels Viggo Bentzon
http://www.sheetmusicplus.com/store/...s+Item-_-Title
Romantic Suite by Findeisen. Out of print, but scans may be available somewhere, if you know what I mean. Requires solo tuning.
Frank Proto has a duet. Probably Liben Music
Serge Lancen - concerto (Billaudot)
William Sydeman -Duo (Seesaw Music)
Fritz Leitermeyer - 12 Dialogues (Doblinger)