I am pretty sure the MODX can be programmed to play two octaves right next to each other...?
I mean, by pushing the middle C the MODX can play both the corresponding middle C note and the next upper C note at the same time.
Correct?
Easy way to do that would be to create a two part Performance with the same sound in each Part, and edit one of the two Parts to sound an octave higher (Note Shift +12).
Or you could use the "Pitch Change" effect mixing equal wet and dry signal.
https://www.yamahasynth.com/ask-a-question/montage-6-help-adding-an-octave-while-playing#reply-78112
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Current Yamaha Synthesizers: Montage Classic 7, Motif XF6, S90XS, MO6, EX5R