How to slur across staves in lilypond (not a single voice)?

Question

I'm trying to engrave these phrasing slurs across two staves:

I've read this question, but it seems different because the slurred phrase was a single voice that crossed between staves. In my case, while these parts "sound like" one voice, their durations fit in with the other left and right hand voices.

Is there a way I can add this slur without actually turning it into a single cross-staff voice (which would require hacking some note durations)? Or, if making it one voice is the only solution, what's the cleanest way to do that?

This is what I have so far:

\version "2.22.1"

\parallelMusic rh,lh,pedal {
  \key cis \minor \time 3/4 |
  \key cis \minor \time 3/4 \clef bass |
  |

  <e, gis cis>2 gis8 cis |
  cis,,8\< gis' cis e4. |
  s2\sustainOn s8 s8\sustainOff |

  bis2 bis8 gis' |
  gis,8 dis' fis gis4. <>\! |
  s2\sustainOn s8 s8\sustainOff |
}

\score {
  \new PianoStaff <<
    \new Staff = "upper" \relative c' \rh
    \new Staff = "lower" \relative c' \lh
    \new Dynamics \pedal
  >>
}

Answer 1

You can do it by adding hidden note heads like this:

\version "2.22.1"

\parallelMusic rh,lh,pedal {
  \key cis \minor \time 3/4 |
  \key cis \minor \time 3/4 \clef bass |
  |

  <e, gis cis>2 gis8 cis |
  cis8\<^\shape #'((0 . 0) (0 . 1) (0 . 3) (0 . 3)) \( gis' cis <<
    \new Voice e4.
    {
      s4
      \change Staff = "upper"
      \once\override NoteColumn.ignore-collision = ##t
      \once\omit Stem
      \once\omit Flag
      \once\hideNotes
      cis'8\)
      \change Staff = "lower"
  } >> |
  s2\sustainOn s8 s8\sustainOff |

  bis2 bis8 gis' |
  gis,8 dis' fis gis4. <>\! |
  s2\sustainOn s8 s8\sustainOff |
}

\score {
  \new PianoStaff <<
    \new Staff = "upper" \relative c' \rh
    \new Staff = "lower" \relative c, \lh
    \new Dynamics \pedal
  >>
}