The idea here is to combine the arpeggios of 2 consecutive diatonic chords, so that we end up with all 7 notes of a given scale.
ex. for a C Major scale (or C IONIAN) play CMaj7 (C E G B) with Dm7 (D F A C)
The C note is common to both arpeggios, but indeed, the sum of all 8 notes will add up to the complete C Major scale.
for D DORIAN, play Dm7 (D F A C) with Em7 (E G B D).
What is interesting about the exercise as I present it, is that the first arpeggio is ascending while the second one is descending. The resulting pattern starts -- and ends -- with the tonic of the desired scale (ION or DOR, etc.)
It also makes for a nice technical study.
Note that the 2 consecutive arpeggios may go in either direction. For instance, I demonstrated CMaj7 with Dm7, but a similar effect is attained when we pair up CMaj7 with Bm7(b5)
ex. for a C Major scale again (or C IONIAN) play CMaj7 (C E G B) ascending, and continue descending with Bm7(b5) (B D F A)
The B note is now common to both arpeggios, but, there again, the sum of all 8 notes will add up to the complete C Major scale.
and here are some 1-octave arpeggios for all seventh chords, if you need a refresher for the fingerings: