I actually solved that particular problem by hiding the brackets and numbers, and not using a spacing spanner for the bracket i wish to be visible:
<<
\new Voice {
\voiceOne
\once \override TupletNumber.text = ""
\override TupletBracket.bracket-visibility = ##f
\tuplet 4/3 {
e''2
}
}
\new Voice {
\once \override TupletNumber.text = ""
\override TupletBracket.bracket-visibility = ##f
\tuplet 4/3 {
\voiceTwo
r8 c''4.
}
}
\new Voice {
\voiceThree
\once \override TupletNumber.text = ""
\override TupletBracket.bracket-visibility = ##f
\tuplet 4/3 {
s4 g'4
}
}
\new Voice {
\voiceFour
\tuplet 4/3 {
r4 g'8 d'8
}
}
>>