Squierrel wrote:As far as I know, a double-stop means playing two notes simultaneously instead of one. Again, according to my limited knowledge, the higher of the two notes is the one written in the melody and the lower one is the added spice. Please, correct me if I'm wrong.
It can be
either. It can even be
neither (it is possible to envisage double stopped chords - strictly - "diads" - which avoid the melody notes and present only harmony notes.
My question is: How do I know which note to play as the added lower note?
You need to understand the scales (major and minor) for the key in which you are playing. For a simple three-chord tune in a major key (let's say C), the scales and chords would run:
C Dm Em F G Am Bm C
If you work out your harmonies so as to include adjacent notes out of the triads for those chords, you'll be on the right lines.
HTH,
JN