\setuplayout
  [alternative=doublesided,
   margin=20mm,
   topspace=10mm,
   bottomspace=10mm,
   height=fit,
   width=fit,
   marking=on,
   location=middle,
  ]

\setuppapersize [A4] [doubleoversized, landscape]
\setuparranging [2UP]

\setupbackgrounds
[page]
[background=header,offset=3mm]

\definelayer
  [header]
  [x=-3mm,
   y=-3mm,
   width=\dimexpr\paperwidth+6mm\relax,
   height=\dimexpr\paperheight+6mm\relax,
   doublesided=yes,
   repeat=yes]



\setlayer [header] [left] [preset=lefttop] {\externalfigure[hacker.jpg][height=10mm,width=\dimexpr\paperwidth+6mm\relax]}
\setlayer [header] [right] [preset=righttop] {\externalfigure[hacker.jpg][height=10mm,width=\dimexpr\paperwidth+6mm\relax]}



\starttext

\input tufte

\page [yes]

\input knuth

\stoptext