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>>>...However, when I activate the "Dual" button next to the "Harmony" button, I DO hear two voices...>>>

The DUAL button add another voice to the one you see displayed, playing the same note with a different instrument. If you play a C4 with your right hand, both voices will be playing C4, and there will be two different instrument sounds. This is most often called 'Layering."

When you get the Harmony feature going, the DGX will provide one or more notes that are NOT the same as your Right Hand notes.

Hold down the VOICE button for three seconds. You will see the menu change. You now have access to a whole world of submenus that will allow you to control and combine voices in new ways.

Flatfoot sez: Call me when 'Talent-in-a-Box' is ready to ship! -- [8{>

Got some tunes on You Tube:
http://www.youtube.com/user/flatfoot50
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My BiaB lesson site:
http://jdwolfe0.wixsite.com/learnbiab