Each harmonic triad defines this pair.
E.g, the 3M7-6m-2m harmonic triad defines the 3-2 isokratocs , or the even-odds chord arpeggios. 3-5#-7-2=3M7 and 2-4-6-1 =6m7
https://www.youtube.com/watch?v=kuCGvw-N_BA
The idea, of course, behind the pair of isokratic notes differing by an interval of a 2nd in a mode of a scale is that any melody on a mode of a scale can be harmonized with 2 chords: the odd notes chord corresponding to the note number 5 and the even notes chord corresponding to the note 4.
When in a song, which is in a scale and mode, of notes, numbered 1,2,3,4,5,6,7, the isocratic notes are 4-5, then, the bass line, is the notes 2-4-6-1 as long as isocratic note 4 lasts and 1-3-5-7 as long as isocratic note 5 lasts
This bass method, in a 1,2,3,4,5,6,7 mode-scale, is based on the musical theorem, in harmony, that every melody in such a mode can be harmonized with two single 4-chord chords, 1-3-5-7 and 2-4-6-1. So the bass lines are arpeggios on these two four-chord chords. We always talk about diatonic modes or modes in melodic or harmonic minor.
