You can use the overlay environment to place the two images on top of each
other. Then adjust the relative position of the second figure with the
\offset
command.
\useMPlibrary
[dum]
\setupexternalfigures
[width=10cm]
\starttext
\startoverlay
{\externalfigure}
{\offset[x=9cm, y=5cm]{\externalfigure}}
\stopoverlay
\stoptext

EDIT: The reason subsequent text overlays the second image is that the \offset
command does not adjust the bounding box, which is mostly what is desired. You can visually trace that using the \showboxes
command.
To fix this I present two solutions. The first one creates a \vbox
using the higher level command \frame
with a height of image height + vertical shiftwidth / 2 = 12.5cm which creates a box of the correct height, so subsequent text does not overprint the image.
\setuppapersize
[A3]
\useMPlibrary
[dum]
\setupexternalfigures
[width=10cm]
\defineframed
[overlayframe]
[frame=off,
offset=overlay,
height=12.5cm,
align=top]
\starttext
\overlayframe{%%
\startoverlay
{\externalfigure}
{\offset[x=9cm, y=5cm]{\externalfigure}}
\stopoverlay}
\input knuth
\stoptext

The second solution inserts vertical space by using a \vskip
.
\setuppapersize
[A3]
\useMPlibrary
[dum]
\setupexternalfigures
[width=10cm]
\starttext
\startoverlay
{\externalfigure}
{\offset[x=9cm, y=5cm]{\externalfigure}}
\stopoverlay
\godown [\dimexpr 5cm+\lineheight\relax]
\input knuth
\stoptext
The result is the same as above.