First had Bill play his middle Bb and Gabby the F a fifth above, shifting down to the Eb and back. Once that was established I played the Bb below middle C and Dick played the third and fifth and flat seventh. What really worked was Bill and Gabby playing those Perfect Fifths and Perfect Fourths, because they could really hear the coherence when they were right. My guess is they never really realized fourths and fifths are more consonant than thirds.
So the place to start with the etude is with the fifths and fourths, because I think they're easier to hear and recognize as being in tune than unisons and octaves. There's almost a sense of feeling the standing waves in the air, that octaves don't generate, at least to me. Another way of putting it is that the fourths and fifths suggest triangulation when focussed, whereas octaves are more like lining up points in a straight line.