\version "2.15.30"

\score {
  \new StaffGroup << 
    \new Staff \relative c {
      \override Score.BarNumber #'self-alignment-X = #CENTER % center-aligned
%       \bar""
      \clef "bass"
      c1 | c1 | c | c | \break
      c1 | c1 | c | c | \break
      c1 | c1 | c | c | \break
      c1 | c1 | c | c | \break
    }
    \new Staff \relative c {
%       \bar""
      \clef "bass"
      c1 | c1 | c | c | \break
      c1 | c1 | c | c | \break
      c1 | c1 | c | c | \break
      c1 | c1 | c | c | \break
    }
    
  >>
}