Oups, erreur de copier/coller,
donc:
\version "2.18.2"
flautoI = \relative c'''{ a'4\<( c, e f fis\> g e c) a'\<( c, e f fis\> g e c) a'1\!~ a1 }
flautoII = \relative c''{ a'4\<( c, e f fis\> g e c) a'\<( c, e f fis\> g e c) f'1\!~ f1 }
oboeI=\relative c'''{ a2\<( c bes1\>) a2\<( c bes1\>) a1\!~ a1}
oboeII= \relative c'' { f2\<( a e1\>) f2\<( a e1\>) f1\!~ f1 }
nuances = { s1\cresc s s s s4\f r r2 }
\new StaffGroup <<
\new Staff \with { instrumentName = "2 Flauti" }
{ \partcombine \flautoI \flautoII }
\new Dynamics \nuances
\new Staff \with { instrumentName = "2 Oboi" }
{ \partcombine \oboeI \oboeII }
\new Dynamics \nuances
>>
<<
\new Staff \with { instrumentName = "Flauto I" } { \clef "G^8" \flautoI }
\new Dynamics \nuances
>>
<<
\new Staff \with { instrumentName = "Flauto II" } { \clef "G^8" \flautoII }
\new Dynamics \nuances
>>
<<
\new Staff \with { instrumentName = "Oboe I" } { \clef "G^8" \oboeI }
\new Dynamics \nuances
>>
<<
\new Staff \with { instrumentName = "Oboe II" } { \clef "G^8" \oboeII }
\new Dynamics \nuances
>>