mus = \relative c' {
  \override Score.BarNumber.break-visibility = #all-visible
  \set Score.barNumberVisibility = #(every-nth-bar-number-visible 2)
  c1 | c1 | c1 | c1 | c1
  \override Score.BarNumber.self-alignment-X = #CENTER
  c1 | c1 | c1 | c1 | \break c1
  \override Score.BarNumber.self-alignment-X = #LEFT
  c1 | c1 | c1 | c1 | c1
}
\new ChoirStaff <<
  \new Staff { \mus }
  \new Staff { \mus }
>>