One common problem with double-stop intonation is that the fingers do not correctly adjust to changes between major and minor. This is one way to improve accuracy in these changes.
'Explain' the change of interval to your fingers by practising with the same pair of fingers all the way up and down the scale. Chain (overlapping) bowing, where you bow across the shift, helps this process.
C major with just the 1st and 3rd fingers is a good scale to begin with because it repeats exactly the same pattern of intervals on the G and D strings (major, minor, minor, major) as on the D and A strings.
Add variations such as playing loudly on the chords that change between major and minor and playing softly on the chords that repeat the type of interval. You can practise such scales with and without vibrato.
Also practice 6ths in this way, for instance all 3rd and 4th fingers or all 2nd and 3rd or all 1st and 2nd fingers.
Chords & Double Stops - Introduction
- for more on changes of type of interval in double-stop scales
- for The Art of the Etude by Zakhar Bron
Scores - Double-Stop Scales - Same fingers - violin & viola