\startplacefigure [location=none]
%\startsubfloatnumbering% This does not work!
\startfloatcombination [nx=2, ny=1]
\startplacefigure [title=Left, reference=fig:left] \externalfigure \stopplacefigure
\startplacefigure [title=Right, reference=fig:right] \externalfigure \stopplacefigure
\stopfloatcombination
%\stopsubfloatnumbering% This does not work!
\stopplacefigure