How can i achieve this?
You’ll need to use a custom invisible notehead (Lillie recommends two or three text spaces) for the upstem Gb, given the mismatch in size between the visible notehead and the beam above it.
I suspect a hidden tuplet will give more stable positioning than actual grace notes, too, then it’s just a case of giving both Gbs the same Voice Column Index.
Thank you @pianoleo - it worked! I chose the version with a custom invisible notehead. It looks perfect!
My own sense is that this might make the placement of the high Db clearer to the player, but playback of my version is not the same as what your notation should accomplish.
I agree, in your notation it’s clearer that the high Db comes on beat one. On the other hand, with your notation it is not so obvious that the first grace note is a melody note. Of the three editions available to me the bar in question is always written with the half note Gb at the beginning of the bar (Theodor Kirchner, Prelude op. 9, Nr. 14)
Another famous example of this is Chopin Nocturne in F♯ major, bar 8. The thumb plays one melodic note, arpeggiating up to the top melody note, all ostensibly on the same beat, but with grace notes between. Easy enough to write and understand and play, but difficult to explain to computer software. The beat itself is “fattened” by the arpeggio.