RH = \context Staff = upper \notes \relative c
{
   \time 4/4
   c'4 e g c
   \property Staff.whichBar = "|."
}

LH = \context Staff = lower \notes \relative c
{
   \time 12/8
   c'8 d e d e f g a b a b c
   \property Staff.whichBar = "|."
}

\score
{
   \context PianoStaff
   <
      \RH
      \LH
   >

   \paper
   {
      \translator
      { 
         \ScoreContext \remove "Timing_engraver" 
      }

      \translator 
      { 
         \StaffContext \consists "Timing_engraver"
      }
   }
}

\version "1.6.4"