Other than setting the stems to the same direction you can also use
\once \override NoteColumn.force-hshift = 1
(or -1, depending on what voice you want first).
A clarification: Voices have settings for stem direction and shift. For the
noteheads not to be merged they need to have the same stem direction and
different shift settings (or no shift setting at all, but then they throw a
warning, like in jJean-Julien’s suggestion). The \voiceXxx commands do not
create or change voice contexts, but only change these settings (and the
directions of articulations and slurs). These settings can also be changed
individually with \stemUp \stemDown and \stemNeutral resp. \shiftOn and
\shiftOff. For instance \voiceOne and \voiceThree sets stems up and \voiceTwo
and \voiceFour down, \voiceOne and \voiceTwo sets shift off and \voiceThree and
\voiceFour set shift on.
HTH
/Leo
> 11 jan. 2023 kl. 16:18 skrev Johannes Roeßler <j...@joei.de>:
>
> Hi Group,
>
> how can I avoid that the same notes in two voices are merged?
>
> \version "2.24.0"
> <<{ a'4 }\\{ a' }>>
>
> Best regards
> Joei
>
