voiceA = \relative { d''4 r d2 | R1 | }
voiceB = \relative { fis'4 r g2 | R1 | }
\score {
  <<
     \new Staff \with {
       instrumentName = "unmerged"
     }
     <<
       \new Voice { \voiceOne \voiceA }
       \new Voice { \voiceTwo \voiceB }
     >>
     \new Staff \with {
       instrumentName = "merged"
       \consists "Merge_rests_engraver"
     }
     <<
       \new Voice { \voiceOne \voiceA }
       \new Voice { \voiceTwo \voiceB }
     >>
  >>
}