I am making some progress with a bilingual text using the streams 
mechanism suggested by Wolfgang: https://wiki.contextgarden.net/Columns, 
but see below.

The aim has been to have en and vi on facing pages (en left, vi right).

It works correctly for the Foreword in two languages (en, vi) in 
frontmatter.

When I come to bodymatter and Chapter 1, again it works, except that the 
positions of the languages are reversed, so I end up with vi on the left 
and en on the right. By Chapter 2 we are back to the correct position 
once more (en left and vi right).

I want to know why this is happening, and if I can prevent it happening.

I am using the code you can see below, except that instead of 
\startoutputstream[one] and \startoutputstream[two] etc.  I am using 
[en-front][vi-front], and then in bodymatter [en-chap1][vi-chap1] etc. 
etc. with the relevant repetitions of these identifiers where needed. I 
assume that to get a degree of synchronization I need to use this 
approach for each set of chapters. But I cannot understand why the 
languages alternate their starting page positions each chapter.

There are other issues I have to solve, but first of all I wish to 
resolve this one.

Julian


The original (Wolfgang) code is as follows:

\starttext

\startoutputstream[one]
    \startcolor[red]
    \dorecurse{10}{\input  knuth\par}
    \stopcolor
\stopoutputstream

\startoutputstream[two]
    \startcolor[green]
    \dorecurse{10}{\input  zapf\par}
    \stopcolor
\stopoutputstream

\synchronizestreams[one,two]

\setbox\scratchboxone\outputstreambox[one]
\setbox\scratchboxtwo\outputstreambox[two]

\doloop
  {\ifvoid\scratchboxone
     \exitloop
   \else
     \setbox\scratchboxfour\vsplit\scratchboxone  to\textheight
     \vbox  to\vsize{\box\scratchboxfour\vss}%
     \setbox\scratchboxfive\vsplit\scratchboxtwo  to\textheight
     \vbox  to\vsize{\box\scratchboxfive\vss}%
   \fi}

\stoptext