Watch the E add 9, C add 9 Vamp online guitar lesson by Brad Carlton from Guitar Lab: The Golden Arp
This lesson will apply various add 9 voicings to the chord progression E add 9, C add 9. We'll discuss common tones, voice leading, and the concept of using all four inversions of a particular voicing.
The common tone between these two chords is the E. It is the root of the E add 9 and the 9(2) of the D add 9. Other "moves" would be B up to C, G# down to G, E down to D, G# up to A, and E up to F#. Be sure to explore all of these in your soprano tone when moving from the E add 9 to the D add 9.