\version "2.12.2"

global = {
  \key aes \major
  \time 3/4
}

upperA = \relative c'' {
  \time 3/4
  bes4 f4. g8
  bes4( aes2)\fermata
  
}

upperB = \relative c' {
  \stemDown
  \override NoteColumn #'ignore-collision = ##t
  \override Stem #'flag-style = #'no-flag
  \tieUp
  \stemDown
  f2.
  %% COLISION
  \once \override NoteColumn #'force-hshift = #-1.0 ees2.

}

upper = {
  \global
  \clef treble
  << \upperA \\ \upperB >>
}


lowerA = \relative c' {
  \change Staff = "upper"
  \stemDown
  \tieDown\slurDown des2.~ 
  %% COLISION
  des4( c2)_\fermata
  
  \hideStaffSwitch
  \change Staff = "lower"
}

lowerB = \relative c {
  g8[ aes] bes4 ees4 ~ << { ees2.\fermata }\\{ aes,2._\fermata} >> 
}

lower = {
  \global
  \clef bass
  << \lowerA \\ \lowerB >>
}

\score {
  <<
    \new PianoStaff <<
      \new Staff = "upper" \upper
      \new Staff = "lower" \lower
    >>
  >>
  \layout {
  }
}