When decorative non-chord notes (e.g. passing notes or auxiliary notes) are used in two or more parts at the same time, it’s best to make sure that the decoration notes sound good together. The easiest way to achieve this is to write the decoration notes at an interval of a 3rd or a 6th apart.
Here, decoration notes are added in the soprano and alto. Both are passing notes.
Avoid 4ths and 5ths, as these can sound very dissonant or harsh, for example this 4th created by passing notes in the alto and tenor parts:
Or this 5th, created by auxiliary notes:
Decoration notes can be used in any number of parts, to create a completely new chord, called a “passing chord”. Bach often uses passing chords in his Chorales.
a) auxiliary and passing notes = 3rd
b) passing notes = 6th
c) auxiliary and passing notes = chord iiib (passing chord).