One and a half voice

[Helge Kruse]

Hello,

I want to write a fragment as shown in the attachment. I wrote it as

        \clef treble
        \time 2/4
        \key f \major
        << { f8 a g a  f a g a  f a g a g b <g a>4 } \\
        { d4 e  d e  d e  d c } >>

This gives the same notes but the stem does not connect the notes. The 
lower voice in introduced only for a few measures and indicates that 
these notes are to be played with a bit accent. So it's not really a 
voice by it's own.

How would you write it in Lilypond to get closer to the original?

Regards,
Helge