\version "2.13.18"

upper = \relative c' { \clef treble c1 } 
lower = \relative c { \clef bass c1 }
dynamics = { s1\mf }

\score {
  \new PianoStaff {
    <<
      \new Staff { \upper }
      \new Dynamics { \dynamics }
      \new Staff { \lower }
    >>
  }
  \layout {}
}