\version "2.18.2"

%Parts that should be quoted
stimmeI = \relative d'' { d1 | c1 }
stimmeII = \relative g' { g1 | a1 }

\addQuote "stI" { \stimmeI }
\addQuote "stII" \stimmeII

%Parts, where the first note is not quoted, 
%		the second is quoted from above
instrumentI = \relative b' {
  b1 |
  \quoteDuring #"stI" { s1 }
}

instrumentII = \relative f' {
  f1 |
  \quoteDuring #"stII" { s1 }
}

% \partcombine works how it should for the parts without quotes.
\score {
    \partcombine \stimmeI \stimmeII
}

% quoting works how it should, when not using partcombine.
\score {
  \new Staff <<
    \instrumentI \\
    \instrumentII
  >>
}

% quoting for part I doesn't work when using \partcombine:
% (in more complex examples even both parts are missing).
\score {
  \partcombine \instrumentI \instrumentII
}