Tie across voices

[David Sumbler]

\version "2.19.81"

\new Staff {
  \time 6/8 
  <<
    { \once \override MultiMeasureRest.staff-position = #10
      R2. | r4. e'''~ }
    \\
    { g''2.~ | g''~ }
  >>
 \oneVoice <e''' g''>2.~ | q |
}

In the above I want the lower note to be tied continuously.  As it
stands, there is no tie between the 2nd and 3rd bars where the 2 voices
become one.

I have experimented a lot with invisible notes (as suggested in section
4.7.1 in the Learning Manual) and with additional voices, but I haven't
yet managed to get the result I want.

This is taken from a full score where the violins are divisi.  The
notes continue for a further 10 bars, so I don't want to continue with
up and down stems, which would look very untidy.

Can somebody suggest how I can get the effect I want?

David