Hello.
I can't find a way to have a single sequence of dynamics that affects both upper and lower piano staves in the MIDI output.
I tried the following snippet but I can't hear any variation of the dynamics in the midi file.
%%%%%%%%%%%%%%%%%%%%%%%%%%%
\score {
<<
\new PianoStaff <<
\new Staff="up" { c'4 c' r r }
\new Dynamics { s4\fff s s\ppp s}
\new Staff="down" { r4 r c' c' }
>>
>>
\layout { }
\midi { }
}