Coordinated fast passagework across two strings relies on the fingers on both strings being ready in time. Also when playing at fast tempos the ear will often perceive individual notes as part of a chord.
Find and practise the implied double-stops in such passages.
Tune each pair of fingers together to make a double stop. Then place the fingers at exactly the same time, with clear articulation.
Using pizzicato or a martelé bowstroke helps the left-hand fingers to articulate the notes with a clear fingerfall.
- for Concerto for Violin No.3 by Saint-Saens, 1st movement, Bar 271