Hello Yoann,
Maybe starting this way:
\version "2.19.83"
{
% initial shared music
c'1 e |
% the two alternatives
\override Staff.StaffSymbol.line-count = 0
\stopStaff \startStaff
\hide Staff.BarLine
<<
\new Staff {
\hide Staff.Clef
\hide Staff.KeySignature
\h
Hello everybody out there!
I must write a score for several instruments. In this score, interprets
must several times choose between two alternatives. To symbolize these
choices, the staff is split in two, every staff being linked to the
other with an opening angle bracket (“<”). At the
