\version "2.15.12"

\layout {
  \context {
    \Voice
    \consists
      #(lambda (context) `((listeners . ((note-event . ,(lambda (eng ev) #t))))))
    % This works, because it does not create a copy for each voice:
%     \consists
%       #'((listeners . ((note-event . ,(lambda (eng ev) #t)))))
  }
}

\relative c'' { <<
  { c4 } \\ { c4 }
>>
}