Turn off merge-differently-headed for one note

[Nick Payne]

I have a guitar piece in two voices in which I'm using 
\mergeDifferentlyHeadedOn in the \score section to merge the same notes 
in the two voices. However, there are a couple of places where I want 
the same note played on two different strings and therefore don't want 
the notes merged. I tried using

\once \override Staff.NoteCollision #'merge-differently-headed = ##f

before the relevant notes in both voices, but those notes are still 
getting merged. Is it possible to override the \mergeDifferentlyHeadedOn 
for individual notes? In the example below, how can I prevent the first 
note in each voice from being merged?

%============================================
\version "2.13.26"

\relative c'' {
    \new Staff <<
      \new Voice = "first"
        { \voiceOne \once \override Staff.NoteCollision 
#'merge-differently-headed = ##f b8 b b b b b b b }
      \new Voice= "second"
        { \voiceTwo \once \override Staff.NoteCollision 
#'merge-differently-headed = ##f b8 b b b b b b b }
>>
}
%============================================


Nick