Although my celtic/folk strings sheet music is all scored for a flexible string quartet, it's also possible to use all of my arrangements for playing celtic fiddle duets.
You can do this in two different ways.
The first way is the simplest - one person plays the violin 1 part, and the other person picks either the violin 2 or the violin 3 part, depending on their preference.
The second way is more complex, and it allows you to unleash your creativity and build an arrangement where you play the tune multiple times through, as you might do in a folk band or fiddle group. What you can do is have parts 1 and 2 the first time through, then swap to playing parts 1 and 3. This adds a bit of variety to the music, and also means that if you want, you and your duet partner can take turns with who plays the melody and who plays the harmony.
If you have three or more people, of course, you can use a similar technique in building trio or fiddle band arrangements, but I thought I'd focus on the fiddle duets in this post as it's not instantly obvious that my four-part arrangements are actually a great basis for playing duets!