Getting voices to merge in a complex measure

[Knute Snortum]

Hello everyone,

I'm engraving Chopin's Prelude No. 1 in C and I'm having trouble
getting two voices to merge.  It's the two lower d's in the last 16th
of the measure.  I've attached the source code and a picture of what
it looks like.  The second picture is how I would like it to look.  I
have notes in the source code that document some of what I discovered
fooling around with the notes.

Any help would be appreciated.

--
Knute Snortum

\version "2.22.2"

staffUp   = \change Staff = "upper"
staffDown = \change Staff = "lower"

beamPositionsC = \once \override Beam.positions = #'(2.5 . -1)

rightHandUpper = \relative c' {
  \voiceOne
  \scaleDurations 2/3 {
    % Change d16 to s16 and the other two d's in the rightHandLower 
    % and leftHandUpper merge
    r16 e a e'8 d16 |
  }
}

% Last two lower d's don't merge
rightHandLower = \relative c'' {
  \voiceThree
  \scaleDurations 2/3 {
    s8. \beamPositionsC c16 a d, |
  }
}

% This way merges the last two lower d's, but looks odd
%{
rightHandLower = \relative c'' {
  \voiceFour
  \scaleDurations 2/3 {
    s8. \staffUp c16 a d, |
  }
}
%}

rightHand = <<
  \clef treble
  \time 2/8
  \mergeDifferentlyDottedOn
  \new Voice \rightHandUpper
  \new Voice \rightHandLower
>>

leftHandUpper = \relative c' {
  \voiceFour
  \scaleDurations 2/3 {
    s16 \staffUp e8.*4/3[ d16] |
  }
}

leftHandLower = \relative {
  \voiceFour
  \scaleDurations 2/3 { f,16 c' a' } r8 |
}

leftHand = <<
  \clef bass
  \time 2/8
  \mergeDifferentlyDottedOn
  \new Voice \leftHandUpper
  \new Voice \leftHandLower
>>

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