Naming a whole GrandStaff

You can have a name for the whole GrandStaff in addition to individual Staffs.

\version "2.24"

%% http://lsr.di.unimi.it/LSR/Item?id=115

\score {
  \context GrandStaff <<
    \context Staff = "treble" {
      \set GrandStaff.instrumentName = "Violini  "
      \set Staff.instrumentName = " vn I" { c''4 }
    }
    \context Staff = "bass" { \set Staff.instrumentName = " vn II" c''4 }
  >>
}