Okay Taco, so that's a possibility I can explore, even though it is not a nice one, as you say. I can think of a couple of other not-so-nice ways of achieving a result as well. The manuscript shown me (in MSWord, obviously) has been carefully prepared to ensure no text flows over a page, so we are really dealing with separate pages. But of course, all that changes once page dimensions and font size etc. get involved at the ConTeXt stage.
However, you gave me a clue: 3 files: "by creating two separate pdfs for the page bodies only and using a third context doc to include those pages alternated'.
For the first chapter of the proposed book I have tried Wolfgang's suggestion as listed in https://wiki.contextgarden.net/Columns#Streams by simply using the \input method. Obviously 'en' and 'vn' stand for two tex files set up as I normally would with preamble and bodymatter. I certainly get a good result once the chapter starts. It would seem logical that further chapters can be added. Obviously I only want one TOC, not two (only call it in for, say, the en.tex but not for vn.tex) and for some reason I have several blank pages before everything starts... but hopefully can solve that. Maybe I can adapt what we have below with the project structure.Not ready to give up yet!
Julian
\starttext
\startoutputstream[one]
\startcolor[red]
\input en\par
\stopcolor
\stopoutputstream
\startoutputstream[two]
\startcolor[blue]
\input vn\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
Way, way back when, I did a document in NL + EN by creating two separate pdfs for the page bodies only and using a third context doc to included those pages alternated, with the correct headers added in that third document. Not a ’nice’ solution at all, but it worked for me. (this was in the nineties, though)