The Dual Mode; Other Dual Mode Functions - Yamaha Clavinova CLP-555 Owner's Manual

The Dual Mode

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To activate the dual mode simply press two voice selectors at the
same time (or press one voice selector while holding another). The
voice indicators of both selected voices will light when the dual mode is
active. To return to the normal single-voice play mode, press any single
voice selector.
The CLP-555 Function mode provides access to a number of other
dual-mode functions, listed below. See the corresponding pages for
details.