\stoptext
</texcode>
Reference: http://www.ntg.nl/pipermail/ntg-context/2011/063030.html
Or, another solution (which can be tuned as will), which relies on a
new feature in MkIV: there is also a layout with the same name as the
makeup environment and you can control the margins of your new title
page environment with \setuplayout[makeupname][...].
<texcode>
\setuppagenumbering[alternative=doublesided]
%\showframe
\setuppapersize[A5][A4,landscape]
\setuparranging[2UP]
\definemakeup[titlepage]
\setuplayout[titlepage]
[backspace=2cm,width=108mm] % 148mm is the width, 20 mm of backspace,
% 20 mm of outer margin
\starttext
\starttitlepagemakeup
\raggedcenter
\bfc This should be the long, long, very long title\par
\stoptitlepagemakeup
\dorecurse{20}{\input tufte\par}
\stoptext
</texcode>