> You need an additional \once \stemDown in my
> version as well. The way you create the polyphonic section (separating
> the voices with \\) will implicitly make the first voice \voiceOne and
> the other \voiceTwo.
> So what your example does is: create a third with stems up and hide it.
> But as it is stems up the note will be doubled. If you make that
> stemDown the visible and the hidden note heads will merge.