Play the lower note, and while sustaining it, gently drop the upper finger.

To achieve a high-quality sound when practicing double stops, you must look beyond the written page. Fischer’s pedagogy emphasizes several foundational elements that must align perfectly: 1. The Acoustic Phenomenon of Difference Tones

Training the ear to listen for sympathetic vibrations and resultant tones to ensure perfect intonation. 2. Anatomy of a Perfect Double Stop