Yamaha made a big mistake using the terms "Duo" and "Dual" to describe modes in the same product. It is certain to confuse people, especially considering the model could be a beginner's first digital piano.
Duo meaning: two players, same piano, same octave range, side by side
Dual meaning: layering two voices
Much more sensible, Casio refers to these as "Duet" and "Layer" respectively.
Before this goes _completely_ off the rails:<g>
Casio uses three words:
. . . "Split" -- the keyboard is split, left side / right side. One "tone" (= "patch" = "voice") on the left side, and one or two (see "Layer") on the right side.
. . . "Layer" -- two "tones" sound together when a key is pressed. The _right_ side of a split keyboard can also be layered -- for example:
. . . left side: Acoustic bass
. . . right side: Grand Piano + Strings
. . . "Duet" -- the left side and right side use the same "tone" (only one, no layering), and each half-keyboard is shifted an octave or two, so that they play similar pitches.
In "Split" mode, the damper pedal affects the whole keyboard.
In "Duet" mode, there's a separate damper pedal for each half of the keyboard. Half-pedalling work on the right-hand pedal of the 3-pedal set.