\version "2.19.83"

musica = \relative c'{ f4 g << { \hideNotes a4.( \unHideNotes g8) } \\ { a4.( g8) } >> }
musicb = \relative c'{ f4 g << { a4.( g8) } \\ { a4.( g8) } >> }

\score {
  \musica
}
\score {
  \musicb
}