\version "2.19.84"

\markup "Default accidental combining:"
\score {
  \new Staff <<
    \clef bass \key ges \major \omit Staff.TimeSignature
    \new Voice { \voiceOne <b e' fis'>4 }
    \new Voice { \voiceTwo b2 }
  >>
}

\markup "Desired output (by changing notes):"
\score {
  \new Staff <<
    \clef bass \key ges \major \omit Staff.TimeSignature
    \new Voice { \voiceOne <b e' fis'>4 }
    \new Voice { \voiceTwo bes2 }
  >>
}

\markup "Desired output (by applying tweaks):"
\score {
  \new Staff <<
    \clef bass \key ges \major \omit Staff.TimeSignature
    \new Voice { \voiceOne <
      \tweak Accidental.extra-offset #'(-1.3 . 0) b
      e'
      \tweak Accidental.extra-offset #'(0.7 . 0) fis'
      >4 }
    \new Voice { \voiceTwo \once\omit Accidental b2 }
  >>
}