Two bookparts one the same page

[Pierre Perol-Schneider]

Hi List,
As a workaround to set the system-system spacing of two sores on the same
page, I was thinking of this -- bad -- hack:

\version "2.20.0"

un = { \repeat unfold 16 s1 \bar "|." }
deux = { \repeat unfold 8 s1 \bar "|." }

\book {
  \header {
    title = "Titre"
  }
  \bookpart {
    \markup\bold\fontsize #2 "Un"
    \score  {
      \un
      \layout {
        system-count = 4
        \context {
          \Score
          \override BarNumber.Y-extent = #'(0 . 15)
        }
      }
    }
    \markuplist { \vspace #5 \bold\fontsize #2 "Deux" }
    \score  {
      \deux
      \layout {
        system-count = 2
        \context {
          \Score
          \override BarNumber.Y-extent = #'(0 . 20)
        }
      }
    }
  }
}

However, the override do not apply on the 2nd systems.
Any idea why?
TIA, cheers,

Pierre