Urs Liska <address@hidden> writes:
I was trying to reproduce the example .png given in the blog,
accessible at http://felixrosch.com/example.png.
That example shows two grouped staves, each with two horn parts
written on the same stem, as though they were a chord.
Ah, that makes it clearer.
If you want to achieve that result but want (obviously) to have
separate voices
I don't see anything here that would warrant separate voices. Just use
\new Voice << \tromboneOne \tromboneTwo >>
in order to put both into the same voice and be finished.