\useMPlibrary[dum]
\starttext
\startplacefigure[location=,reference=fig:gateNOT,title=]
\startcombination[nx=2,ny=1]
{externalfigure[dummy][width=.25\textwidth]}{a}
{externalfigure[dummy][width=.25\textwidth]}{b}
\stopcombination
\stopplacefigure
\stoptext
Log file:
metapost > library 'dum' is loaded
fonts > preloading latin modern fonts (second stage)
fonts > 'fallback modern rm 12pt' is loaded
tex error > tex error on line 5 in file /Users/graph/Library/Mobile Documents/com~apple~CloudDocs/Documents/test.tex: Missing number, case 7, treated as zero
<to be read again>
]
<line 3.5>
{externalfigure[dummy][width=.25\textwidth]
}{a}
1 \useMPlibrary[dum]
2 \starttext
3 \startplacefigure[location=,reference=fig:gateNOT,title=]
4 \startcombination[nx=2,ny=1]
5 >> {externalfigure[dummy][width=.25\textwidth]}{a}
6 {externalfigure[dummy][width=.25\textwidth]}{b}
7 \stopcombination
8 \stopplacefigure