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* Use the Split mode. Make sure the sequencer is not enabled unless you want that.
* The lower zone plays sound A. The upper zone can morph between A and B, so you may want to set Morph all the way to B.
* In Settings/MIDI/Channels you can set the lower part channel separately from the main (and upper) channel.
You can read more about using the Split mode in the manual chapter "5.4.2. Timbrality".