"exploding" a choirStaff

[Michiel Lange]

I have this piece of music I'm working on, and for the greater part the 
music of the Sopranos and Altos are combined on a Staff with the usage 
of VoiceOne and VoiceTwo. The same goes up for the Tenors and Basses.

Almost at the end of the piece I wanted to "explode" the combined voices 
into separate staves, and thought I would solve it this way in the Score 
(see \score part below). However, Lilypond starts to cross fingers, 
wants to go back in time and another bunch of errors... Obviously, I am 
doing something utterly stupid... but what is it?

Thanks in advance!
- Michiel

--> score snippet <--
\score
{
       << {
       \context ChoirStaff<<
               \context Staff = women <<
               \key ges \major \clef violin
               \context Voice= sop
               {
                       \relative c''
                       {
                               \voiceOne << \sopm >>
                       }
               }
               \context Voice=alt
               {
                       \relative c''
                       {
                               \voiceTwo << \altm >>
                       }
               }
               >>
               \lyricsto sop \new Lyrics \lyrenw
               \context Staff = men <<
                       \clef bass \key ges \major
                       \context Voice=ten
                       {
                               \relative c'
                               {
                                       \voiceOne << \tenm >>
                               }
                       }
                       \context Voice=bas
                       {
                               \relative c'
                               {
                                       \voiceTwo << \basm >>
                               }
                       }
                       \lyricsto ten \new Lyrics \lyrenm
               >>
       >>
       \context ChoirStaff
       <<
               \context Staff=sopamen
               <<
                       \key ges \major \clef violin
                       \context Voice=amensop
                       {
                               \relative c''
                               {
                                       \amensopm
                               }
                       }
                       \lyricsto amensop \new Lyrics \amensopl
               >>
               \context Staff=altamen
               <<
                       \key ges \major \clef violin
                       \context Voice=amenalt
                       {
                               \relative c'
                               {
                                       \amenaltm
                               }
                       }
                       \lyricsto amenalt \new Lyrics \amenaltl
               >>
               \context Staff=tenamen
               <<
                       \key ges \major \clef violin
                       \context Voice=amenten
                       {
                               \relative c'
                               {
                                       \amentenm
                               }
                       }
                       \lyricsto amenten \new Lyrics \amentenl
               >>
               \context Staff=basamen
               <<
                       \key ges \major \clef bass
                       \context Voice=amenbas
                       {
                               \relative c,
                               {
                                       \amenbasm
                               }
                       }
                       \lyricsto amenbas \new Lyrics \amenbasl
               >>
       >> }
       \context PianoStaff <<
       \context Staff
       {
               \relative c'
               {
                       \key ges \major \clef violin
                       \pianor
               }
       }
       \new Staff
       {
               \relative c'
               {
                       \key ges \major \clef bass
                       \pianol
               }
       }
       >>
>>
--> end of score snippet <--