Hi Michiel,
Use shorten-pait to taste.
%\once \override TupletBracket.positions = #'(3.1 . 3.4)
\once \override TupletBracket.shorten-pair = #'(0 . 0.5)
\tuplet 3/2 {
r32 \clef treble
\once \override Slur.positions = #'(-1 . 1)
f'^(-[ f')]
}
%\once \override TupletBracket.positions = #'(3.4 . 3.4)
\tuplet 3/2 { r32 r \clef bass \stemNeutral <f,,,,_~ f'^~> }