It sounds a lovely idea Elizabeth.
My daughter has a book called 'Lollipop Man' easy duets for strings. The pieces are designed for any combination of violins, violas and cellos and are about grade 1 - 2 standard. You need to buy the right book(s) for the instrument(s) you are using - we have the viola book. The format is rather geared towards children however and would be a bit hard for a complete beginner.
Alternatively, you might find something on sibellius for free. Try an advanced search for a mixed ensemble containing the instrument viola for easy music grade 1-3 and choose 'free' (I really feel there ought to be a way of finding specific instrument combinations more easily than this - perhaps someone else knows how). Pick out the ones for violin and viola and have a look at the scores. You can hear what they sound like if you install the scorch plug-in and print off any that are the right difficulty and you like. I came up with the following (but again the viola line might take some work by you!):
http://www.sibeliusmusic.com/cgi-bin/show_...l?scoreid=64026