\version "2.17.0"

% in this example, the part-combiner detects the second beat as SoloII, even though we force it to use separate voices:
IVDuettFagIMusic = \relative f'' {
  \partcombineApart R2 
  \partcombineAutomatic f2
}
IVDuettFagIIMusic = \relative f'' {
  \partcombineApart f4( es4) 
  d2
}
\new Staff {
  \partcombine \IVDuettFagIMusic \IVDuettFagIIMusic
}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% WORKAROUND: Explicitly force the second quarter as Apart, too. This is cumbersome and should NOT be required:
IVDuettFagIIMusicWorkaround = \relative f'' {
  \partcombineApart f4( \partcombineApart es4) 
  d2
}

\new Staff {
  \partcombine \IVDuettFagIMusic \IVDuettFagIIMusicWorkaround
}