This is more a notation question than a lilypond question.
Does exist a standard fingering notation to indicate that a note can be played for example with 2 or 3? I normally write
\finger "2(3)"
or
\finger "2/3"
but I'm not sure this is correct.
I think I've usually seen, for a single note, the "primary", or preferred, fingering on top, with the second fingering directly below :
2
3
That's what I've seen, especially in exercise and etude books, and what I've used in transcribing them. I've also seen, for a single note, one fingering above the staff and another below the staff.
HTH,
Ralph
Ralph Palmer
Brattleboro, VT
USA