\version "2.13.3"
\pointAndClickOff

voiceI = \relative c'' {
  c4( d4) e2~ |
  e1
}

voiceII = \relative c'' {
  c2 b2( |
  c1)
}

lyr = \lyricmode { A __  B __ }

<<
  \new Voice ="v1" \voiceI 
  \new Lyrics \lyricsto "v1" \lyr
  \new Voice ="v2" \voiceII
  \new Lyrics \lyricsto "v2" \lyr
>>

% Question: How can I (automatically!) insert the lyrics with the correct rhythm?
<<
  \new Staff \partcombine \voiceI \voiceII
  \new Devnull="nowhere" \voiceI
  \new Lyrics \lyricsto "nowhere" \lyr
>>