\version "2.13.42"

%% Goal: placing a partial measure in the middle of a score

%% Single Voice: it works!
\new Staff \relative c' {
  \time 4/4
  R1 |
  \set Timing.measurePosition = #(ly:make-moment -2 4)
  c2 |
  c4 d e f |
}

%% Polyphony > warning: barcheck failed...

first = \relative c'' {
  \time 4/4
  c2 c |
  % If I comment the following line, it works (no warning)
  \set Timing.measurePosition = #(ly:make-moment -2 4)
  c2 |
  c1 |
}

second = \relative c' {
  \time 4/4
  c2 c |
  \set Timing.measurePosition = #(ly:make-moment -2 4)
  c2 |
  c1 |
}

\new Staff <<
  \new Voice {
    \voiceOne {
      \first
    }
  }
  \new Voice {
    \voiceTwo
    \second
  }
>>