"Choose the most logical note to tune to an open string. Leave this finger stable and tune the other note to it. Take as long as it needs to find the maximum combination tone. Lift and place the fingers together."
Choose either the top or bottom note to tune first, depending on the key and the appropriate open strings. Perfect intervals (4ths, 5ths, 8ves) are always good to tune with open strings, so in A major you can tune all A's, D's and E's to the open A.
In this double-stop of C# on A-string and A on E-string the most logical note to tune first is A, which makes an octave with open A.
Play with long bows, aim for a clear sound and take as long as you need to get the best resonance. Use small, gentle finger movements to tune the finger carefully to the open string. Then play the fingered note on its own to check resonance and to retain it in your inner hearing.
Once this note is settled, it becomes the stable note and we tune the next fingered note to that.
When tuning a double stop we need to listen for the combination tone (see next item). When the chord is in tune, lift and place the fingers together three times to reinforce the finger position. You can also do the first two finger-taps silently and only sound out the third finger-tap.
Using a little vibrato when the notes are in tune can help release any tightness in the hand.
Chords & Double Stops - Introduction
Scores - Three-Way Check - violin & viola