How do I merge a quarter-note with the second eighth-note of a pair?

[Kevin Cole]

I think an example will make the question clearer:  How do I merge the
B quarter-note with the B eighth-note following the D eighth-note
below?

%%%%%%%%%%%%%%%%%%%
\version "2.20.0"
\language "english"

\layout {
  \autoBeamOff
}

global = {
  \time 4/4
  \mergeDifferentlyHeadedOn
}

melody = {
  \relative {
    \global

    <<
      { \voiceOne b'4 }
      \new Voice
      { \voiceTwo d,8[( b'8]) }
    >>
    \oneVoice
    a8. g16 g4 r8 g8   |
    \bar "|."
  }
}

\score {
    \new Staff { \melody }
}
%%%%%%%%%%%%%%%%%%%