Need help with \force-hshift or \shiftO... (off, on, onn, onnn, etc.)

[Kevin Cole]

In my more-than-minimal example, the b16 from the first voice gets
merged with the b8. from the second voice. When I tried to create a
minimum (non-working) example, the two didn't merge. But, what I want
below is for the b8. from the first voice to merge with the b8. from
the second voice.

In other words, it should look like b16 b8. g16 with the b8. in the
middle having stems going both directions.  Looking at examples that
were over my head, I attempted \override \once \force-hshift in
various places, but didn't accomplish anything.

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

\layout {
  \autoBeamOff
}

global = {
  \key ef \major
  \time 4/4
}

melody = {
  \relative {
    \global
    <<
      { \voiceOne b'16 b8. }
      \new Voice
      { \voiceTwo b8. g16 }
    >>
    \oneVoice
    a8 b8 c4 r8 d8
  }
}

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

P.S. I think I'm interpreting the attached image correctly, but I
could be wrong about that. In any case, that's what I'm trying to
achieve in LilyPond.

min.png
Description: PNG image