\version "2.13.22"

up = \change Staff = "upper"
down = \change Staff = "lower"

one = \relative c {
  d,8_"Same voice for second half of m. 1: longer stems (BAD)" a' f' \up a
  \voiceTwo f' d c a \oneVoice
  | \down g1

  d,8 a' f' \up a
  \voiceTwo f' d c a \oneVoice
  | g1
}

two = \relative c {
  d,8_"Temporary new voice for second half of m. 1: shorter stems (GOOD)" a' f' \up a
  \new Voice { \voiceTwo f' d c a }
  | \down g1

  d,8 a' f' \up a
  \new Voice { \voiceTwo f' d c a }
  | g1
}

\book {
  \score {
    \new PianoStaff <<
      \new Staff = "upper" { s1*4 }
      \new Staff = "lower" { \clef bass \one }
    >>
  }

  \score {
    \new PianoStaff <<
      \new Staff = "upper" { s1*4 }
      \new Staff = "lower" { \clef bass \two }
    >>
  }
}