Can't convince Lylipond to merge different voices with same notes.

[Alexandre Beneteau]

Hello,

Despite having followed (I think) the manuel advices (by setting 
\property Staff.NoteCollision \override #'merge-differently-headed = 
##t), I can't convince Lylipond (1.7.12) to merge different voices with 
same notes...

I join an example source (test.ly) and the png output.

Has anybody an idea ? Am I wrong ? Or is there an issue I've not figured 
out ?

Thanks in advance,

        Alex.

\include "italiano.ly"
\header { tagline = "" } % for proper automatic cropping

% Manual staff switching
gomd = \notes { \translator Staff=md }
gomg = \notes { \translator Staff=mg }

md = \notes \relative do' {
  \time 3/4
  r2.
  \shiftOff
  \gomg <<dod red>>16^\pp \gomd <<sol la>>-. \gomg <<red' fad>> \gomd <<sol 
la>>-.  \gomg <<dod, red>> \gomd <<sol la>>-. \gomg <<red' fad>> \gomd <<sol 
la>>-.  \gomg <<si, dod>> \gomd <<fa! sol>>-. \gomg <<si dod>> \gomd <<fa'! 
sol>>-.
  \gomg <<dod red>>16 \gomd <<sol la>>-. \gomg <<red' fad>> \gomd <<sol la>>-.  
\gomg <<dod, red>> \gomd <<sol la>>-. \gomg <<red' fad>> \gomd <<sol la>>-.  
\gomg <<si, dod>> \gomd <<fa! sol>>-. \gomg <<si dod>> \gomd <<fa'! sol>>-.
}

mg = \notes \relative do' {
  \time 3/4
  \clef "treble"
  r2.
  \shiftOff
  \stemDown \slurDown
  \property Staff.NoteCollision \override #'merge-differently-headed = ##t
  \property Staff.NoteCollision \override #'merge-differently-dotted = ##t
  [<<dod red>>8---( <<red fad>>-- <<dod red>>-- <<red fad>>-- <<si dod>>-- <<si 
dod>>---)]
  [<<dod red>>8---( <<red fad>>-- <<dod red>>-- <<red fad>>-- <<si dod>>-- <<si 
dod>>---)]
}

\score {
  \context PianoStaff=harpe <
  \context Staff=md { \md }
  \context Staff=mg { \mg }
  >
}