breaking up joined triplets

[Hugh Myers]

In the following:

\version "2.13.7"
upper = {
  \stemDown
  \times 2/3 { e,8 e' b } \times 2/3 {gis8 e' b } \times 2/3 { b e b }
  \times 2/3 { e,8 e' b } \times 2/3 {gis8 e' b } \times 2/3 { b e b }
}

lower = {
  s2.
  s2.
}

{
  \time 3/4
  <<
  \new Staff \relative c'' {
    <<
      \new Voice = "1" { \voiceOne \upper }
      \new Voice = "2" { \voiceTwo \lower }
    >>
  }
  \new TabStaff \relative c' {
    <<
      \new TabVoice = "t1" { \voiceOne \upper }
      \new TabVoice = "t2" { \voiceTwo \lower }
    >>
  }
  >>
}

the triplets are connected by a bar--- how do I prevent this?

--hsm