Duo finds from the 19th century
Both the pieces in Louis Spohr's violin school and the duets by Ignaz Pleyel were popular literature for beginners in the 19th century. New editions prove their popularity to this day.

Louis Spohr (1784-1859) was the most famous German violinist, composer, conductor and teacher of his time. His violin school (1832), the Méthode de violon by the Frenchman Baillot (together with Rode and Kreutzer, 1793) and that of the Belgian Bériot (1858) were the outstanding pedagogical works of the time. Because Spohr always accompanied the pupils with his violin when teaching, all the practice pieces in his school are designed as duos - the first part for the pupil, the second part for the teacher - harmonically very stimulating, as Spohr also wanted to convey musical values.
Kolja Lessing has now republished a selection of these in two volumes with Breitkopf und Härtel. Book 1: The twelve first duos, first on empty strings, then in the first position, are all in C major with whole and half notes, often still tied; I wonder whether this difficult key and the long bow strokes are suitable for beginners. The second dozen of Lessing's selections progress from another four in C major, but with more rhythmic movement, to pieces in various major and minor keys that are really worthwhile. Book 2 is a firework of twelve duos covering many technical and musical aspects: two pieces in the 2nd position in C and A flat major, one in the 3rd position in C sharp major, a fast zigzag, a cantilena, three pieces in four parts for two, arpeggio, polacca with staccato volante and as a final bouquet a theme with eleven variations, close to Paganini - certainly not only valuable for study, but also for performance.
The booklets are arranged in such a way that you don't have to leaf through them for the three-page duets. Detailed texts by Spohr (autographs with transcriptions), editor Kolja Lessing and Karl Traugott Goldbach, the director of the Spohr Museum in Kassel, provide interesting information.
The popular beginners' duets by Ignaz Pleyel have now been published by Henle. The booklet contains the Urtext score and two parts with bowings and fingerings. Some of them are unnecessary, but some good suggestions for voluntary excursions into the 2nd and 3rd positions are helpful. The industrious and successful composer, publisher and piano maker Pleyel was compelled to publish easy duets in ascending degrees of difficulty in 1809. The first duet is not the easiest, precisely because it begins with long sustained notes and is in C major, which is not easy for violin beginners. These are light-footed sonatas in two or three movements, which often change to the parallel major or minor key. They are also suitable as sight-reading exercises, for which it is advisable to play from the score.
Louis Spohr: 36 duos for 2 violins from the violin school, edited by Kolja Lessing; booklet 1, 24 easy duos, EB 8976, € 21.90; booklet 2, 12 virtuoso duos, EB 8977, € 24.90; Breitkopf & Härtel, Wiesbaden
Ignaz Pleyel: Six little duos op. 8 for two violins, edited by Norbert Gertsch, HN 1378, € 16.00, G. Henle, Munich