Sorry for resurrecting the thread after months. I ran into a strange bug(?) I think. Using this code in combination with \partCombine confuses Lilypond and the spanner is terminated mid-bar where the invisible full-measure rest of the other part is hidden.
I could tweak the right padding but honestly i'm really tired with huge amounts of manual adjustments in complex scores. Anything almost-automatic is a win for me. Here is a tiny example: \version "2.23.0" #(define (encompass-spanner-duration grob) (let* ((rb (ly:spanner-bound grob RIGHT)) (right-col (ly:item-get-column rb)) (right-neighbor (ly:grob-object right-col 'right-neighbor))) (if (ly:grob? right-neighbor) (ly:spanner-set-bound! grob RIGHT right-neighbor)))) instrumentOne = \relative c' { c4 d e f | R1 | \override Score.TextSpanner.after-line-breaking = #encompass-spanner-duration d'2\startTrillSpan c | b a | b2 g2 | f1 | e1\stopTrillSpan | } instrumentTwo = \relative g' { R1 | g4 a b c | \override Score.TextSpanner.after-line-breaking = #encompass-spanner-duration d2\startTrillSpan c | b a | g2 f( | e) d | e1\stopTrillSpan | } << \new Staff \instrumentOne \new Staff \instrumentTwo \new Staff \partCombine \instrumentOne \instrumentTwo >> Dimitris