\version "2.25.9"

musa = { \dynamicUp c'4\p \repeat unfold 3 { c' }
  \dynamicUp c'4\p \repeat unfold 3 { c' }
  c'4\p \repeat unfold 3 { c' } }
musb = { \dynamicUp s1
  \dynamicUp c'4\p \repeat unfold 3 { c' }
  c'4\p \repeat unfold 3 { c' } }

\score {
  \partCombine \musa \musb
}

\score {
  \partCombine \musb \musa
}

\score {
  \partCombine \musa \musa
}

\score {
  \partCombine \musb \musb
}