https://twitter.com/suitougreentea/status/905087684560822272

forking-chord.ly

  \version "2.19.64"

#(set-object-property! 'stem-fork-position 'backend-type? pair?)
#(set-object-property! 'skip-stem-fork-printing 'backend-type? boolean?)

forkedChord = #(define-music-function
  (lst chord) (list? ly:music?)
  (let* ((note-event-list (ly:music-property chord 'elements)))
    (begin
     (for-each
      (lambda (note lst)
       (set! (ly:music-property note 'tweaks)
             (append 
              (list `((NoteHead . X-offset) . ,(car lst)))
              (list `((Accidental . X-offset) . ,(cadr lst)))
              (ly:music-property note 'tweaks))))
       note-event-list lst)
     #{
      \once \override NoteColumn.before-line-breaking =
        #(lambda (grob)
          (let* ((note-heads (ly:grob-array->list (ly:grob-object grob 'note-heads))))
           (for-each (lambda (note-head-grob lst)
                      (let* ((stencil (ly:grob-property note-head-grob 'stencil))
                             (X-ext (ly:stencil-extent stencil X))
                             (width (interval-length X-ext))
                             (head-position (caddr lst))
                             (offset (ly:grob-property note-head-grob 'X-offset))
                             (stem-grob (ly:grob-object grob 'stem))
                             (stem-width (/ (ly:grob-property stem-grob 'thickness) 10))
                             (direction (ly:grob-property stem-grob 'direction)))
                       (begin
                        (if (and (= direction UP) (= head-position RIGHT))
                            (ly:grob-set-property! note-head-grob 'X-offset (+ offset width (- stem-width))))
                        (if (and (= direction DOWN) (= head-position LEFT))
                            (ly:grob-set-property! note-head-grob 'X-offset (- offset width (- stem-width)))))))
                     note-heads lst)))
      \once \override Stem.stencil =
        #(lambda (grob)
           (let* ((stencil (ly:stem::print grob))
            (X-ext (ly:stencil-extent stencil X))
            (Y-ext (ly:stencil-extent stencil Y))
            (grob-start-y (interval-start Y-ext))
            (grob-end-y (interval-end Y-ext))
            (width (interval-length X-ext))
            (note-column (ly:grob-parent grob X))
            (note-head-grobs (ly:grob-array->list (ly:grob-object note-column 'note-heads)))
            (direction (ly:grob-property grob 'direction)))
   
       (apply ly:stencil-add
         (map (lambda (note-head-grob lst)
               (let* ((note-head-stencil (ly:grob-property note-head-grob 'stencil))
                      (skip (ly:grob-property note-head-grob 'skip-stem-fork-printing #f))
                      (fork (ly:grob-property note-head-grob 'stem-fork-position '(2.75 . 3.5)))
                      (fork-start (car fork))
                      (fork-end (cdr fork))
                      (head-position (caddr lst))
                      (note-head-height (interval-length (ly:stencil-extent note-head-stencil Y)))
                      (staff-position (ly:grob-property note-head-grob 'staff-position))
                      (stem-attachment (ly:grob-property note-head-grob 'stem-attachment))
                      (start-y-raw ((if (= head-position RIGHT) - +) staff-position (* note-head-height (cdr stem-attachment))))
                      (start-y (/ start-y-raw 2))
                      (start-x (car lst)))
                (if skip
                    empty-stencil
                    (grob-interpret-markup grob
                     (markup (#:overlay
                       ((#:path width
                         (cond
                          ((= direction UP)
                           (list (list 'moveto start-x start-y)
                                 (list 'lineto start-x (max (- grob-end-y fork-end) start-y))
                                 (list 'lineto 0 (- grob-end-y fork-start))
                                 (list 'lineto 0 grob-end-y)))
                          ((= direction DOWN)
                           (list (list 'moveto start-x start-y)
                                 (list 'lineto start-x (min (+ grob-start-y fork-end) start-y))
                                 (list 'lineto 0 (+ grob-start-y fork-start))
                                 (list 'lineto 0 grob-start-y)))
                          (else empty-stencil))))))))))
              note-head-grobs lst))))
      $chord
     #})))

\relative {
  \autoBeamOff
  \once \override Stem.length = 10
  \forkedChord #`((-1.5 -2.5 ,LEFT) (1.5 0.2 ,LEFT) (1.5 0 ,LEFT))
  <c'! cis e>8
  \once \override Stem.length = 10
  \forkedChord #`((-3.0 -4.0 ,LEFT) (0 -1.3 ,LEFT) (0 0 ,RIGHT))
  <ces cis d>8
  \once \override Stem.length = 12
  \forkedChord #`((-1.5 -2.5 ,LEFT) (1.5 0.2 ,LEFT) (1.5 0 ,LEFT))
  <\tweak NoteHead.stem-fork-position #'(2.75 . +inf.0) c! cis a>8
  \once \override Stem.length = 10
  \forkedChord #`((-1.5 -2.5 ,LEFT) (1.5 0.2 ,LEFT) (1.5 0 ,LEFT))
  <\tweak NoteHead.skip-stem-fork-printing ##t c! cis e>8
  
  \stemDown
  \once \override Stem.length = 10
  \forkedChord #`((-1.5 -2.5 ,RIGHT) (1.5 0.2 ,RIGHT) (1.5 0 ,RIGHT))
  <c'! cis e>8
  \once \override Stem.length = 10
  \forkedChord #`((-4.0 -5.0 ,RIGHT) (0 -2.5 ,LEFT) (0 0 ,RIGHT))
  <ces cis d>8
  \once \override Stem.length = 12
  \forkedChord #`((-1.5 -2.5 ,RIGHT) (1.5 0.2 ,RIGHT) (1.5 0 ,RIGHT))
  <\tweak NoteHead.stem-fork-position #'(2.75 . +inf.0) c! cis a>8
  \once \override Stem.length = 10
  \forkedChord #`((-1.5 -2.5 ,RIGHT) (1.5 0.2 ,RIGHT) (1.5 0 ,RIGHT))
  <\tweak NoteHead.skip-stem-fork-printing ##t c! cis e>8
}