Add notes to a polyphonic staff style
You can “explode” polyphonic parts, displaying all voices on separate staffs, regardless of staff style settings. (Other voice parameters remain valid.) This makes it easier to add notes using the pointer.
If you add a note to a staff style that uses MIDI channels for voice separation, it’s automatically assigned the corresponding MIDI channel (of the staff that you add the note to). When you’re finished adding notes, turn off the Explode Polyphony setting so all voices are displayed correctly in one staff.
If you add notes to a polyphonic staff without turning on the Explode Polyphony setting, the notes are assigned the MIDI channel set in the Insert Defaults of the Event inspector (but only if that channel is used by one of the voices in the staff).
Display polyphonic parts on separate staffs
Choose View > Explode Polyphony from the Score Editor menu bar (or use the corresponding key command).