Ties between voices?

[James Harkins]

Hi, been away for awhile.

I'm now writing for khaen, a Thai mouth organ. Fascinating instrument. Notation 
is usually single-staff and heavily multi-voiced.

One nut I never cracked in LilyPond: how to tie a note from one Voice to 
another, e.g.

\new Score {
  \new Staff <<
    \numericTimeSignature \time 2/4  % barline, so E must be tied
    \new Voice \relative c' {
      \voiceOne <e g>2 a8 g4.
    }
    \new Voice \relative c' {
      \voiceTwo r4 f4 ~ <e f>2
    }
  >>
}

I need a tie from the e in <e g>2 (voice 1) to the e in <e f>2 (voice 2).

How to restructure this to make it possible?

Thanks.

hjh