Merging half-note chord into a quarter-note chord

[Knute Snortum]

Dear community, 

I would love some help with a problem I'm having getting a chord to have two quarter-notes and two half-notes.  Here is how the measure should look.

​

And here is my minimal (not) working example:

%%% Start

\language "english"

\relative {

  \key f \major

  \mergeDifferentlyHeadedOn

  <<

    { <c' g' bf f'>4( \arpeggio s a''8) \oneVoice r r4 | }

    \\

    { <g, bf>2 * 1/2 e'2. | }

  >>

}

%%% End

Here is a LilyBin link if that's helpful: http://lilybin.com/yklk78/1

Any help would be greatly appreciated.

---

Knute Snortum

(via Gmail)