It does not have the Piccolo 4 tab that the regular Compact has. This adds some spice to the sound. Then it boosts up to a comparable volume. This is a dedicated bass octave. Also, of course, a tab to switch the grey keys between bass and treble.
Red lights show which octaves are being used for bass.
Dolce 8, Principale 8 and Ottava 4. Not much tonal variation can be found on the lower manual. Principale and Ottava tend to dominate when they are being used. There is also a volume knob for the lower manual. The Compact Duo also has vibrato in two speeds and two depths just like the Combo Compact.