'Dual Mode' ( on some keyboards, this is called 'Layer' ) enables two different 'voices' or, instrument sounds, to be played together. For example, a piano voice, and a strings voice, can be 'dualled' to give a pleasing sound. Any combination of instrument sounds available on your keyboard, can be played in Dual mode.
The auto rhythm and accompaniment styles can also be used, when in dual mode, resulting in a very full sound.
Please experiment with different combinations. The results will suprise you!