In choir scores, you often have the score for two voices (e.g., soprano and alto) in one line:

      \new Staff  = "Frauen"<<
         \new Voice = "Sopran" { \voiceOne \global  \soprano }
         \new Voice = "Alt" { \voiceTwo \global  \alto }
      >>

When they both have a pause at the same time with the same length, lilypond will still print two rests in different positions. If you (like me) think this looks weird, here is how you can change it:

soprano = \relative c' { a2 \oneVoice r4 \voiceOne a4 }
alto = \relative c' { a2 s4 a4 }

In one voice, change to only one voice with \oneVoice for the rest and then back to the usual voice, here /voiceOne. If you do the same in the other voice, you will get warnings about clashing notes, so instead of using a rest, use an invisible rest (spacer) with s.

An alternative is the following command which causes all rests to appear in the middle of the line. It should be used inside the \layout block:

   \override Voice.Rest #'staff-position = #0