</texcode>
Posting by Wolfgang to the mailing list(2016-11-27):
<texcode>
{\raise2\strutdp\outputstreambox[two]}
\stopoverlay
\stoptext
</texcode>
The mechanism to create pages/columns for the stream content is
missing.
You can save text and flush it afterwards but there is no command which
places the content side by side (or in columns). Below is a simple example
but I had to write the splitter myself.
</texcode>
\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