combine music, merge noteheads

Bonjour,
(illustration PNG ci-jointe)
A partir de deux expressions musicales, variables "sopone" et "soptwo", comment puis-je obtenir les sorties "desired 1" ou "desired 2" ? Je veux combiner ces deux voix, en ayant les notes identiques affichées une seule fois, et utiliser le tout comme une \voiceOne (avec les hampes vers le haut, de façon à pouvoir rajouter une autre voix "alto" en \voiceTwo). Tout ce que j'obtiens jusqu'ici est illustré ci-dessous.
Merci,
Germain

%% "test-merge-noteheads.ly"

\version "2.11.49"

#(set-global-staff-size 15)

sopone = { c''1 c''2 g'8 a' b' c'' e'' f'' g'' e'' }
soptwo = { c''1 c''2 g'8 f' g' c'' e'' d'' c'' e'' }
\markuplines {
"Dear all,"
"Given "sopone" and "soptwo" (variables),"
"how to get "desired 1" or "desired 2" ?"
}
\score
{
\new GrandStaff
<<
\new ChoirStaff
<<
\new Staff
<<
\set Staff.instrumentName = "sopone"
\new Voice << \sopone >>

\new Staff
<<
\set Staff.instrumentName = "soptwo"
\new Voice << \soptwo >>

\new ChoirStaff
<<
\new Staff
<<
\set Staff.instrumentName = "desired 1"
\new Voice { \voiceOne <c'' c''>1 c''2 g'8 <a' f'> <b' g'> c'' e'' <f'' d''> <g'' c''> e'' }

\new ChoirStaff
<<
\new Staff
<<
\set Staff.instrumentName = "desired 2"
\new Voice { \voiceOne c''1 c''2 g'8 <a' f'> <b' g'> c'' e'' <f'' d''> <g'' c''> e'' }

\new ChoirStaff
<<
\new Staff
<<
\set Staff.instrumentName = "got 1"
\new Voice << \sopone \soptwo >>

\new Staff
<<
\set Staff.instrumentName = "got 1 bis"
\new Voice << \voiceOne \sopone \soptwo >>

\new Staff
<<
\set Staff.instrumentName = "got 2"
\set Staff.printPartCombineTexts = ##f
\new Voice << \partcombine \sopone \soptwo >>

\new Staff
<<
\set Staff.instrumentName = "got 2 bis"
\set Staff.printPartCombineTexts = ##f
\new Voice << \voiceOne { \partcombine \sopone \soptwo} >>

\new Staff
<<
\set Staff.instrumentName = "got 3"
\new Voice { \voiceOne \sopone }
\new Voice { \voiceTwo \soptwo }

\new Staff
<<
\set Staff.instrumentName = "got 4"
\new Voice { \voiceOne \sopone }
\new Voice { \voiceThree \shiftOff \soptwo }

\layout {}
}
%% eof