Thank you Wolfgang.
Follow-up question: how to have a right margin appear on the right side of every booklet page?
The layouts I have tried with the following mwe put the right margin on the cut side of the paper, but that “reverses” the margin side from the reader’s view when printed and folded.
\definepapersize[booklet][width=140mm,height=216mm]
\setuppapersize[booklet][letter,landscape]
\setuppagenumbering[alternative=doublesided]
\setuparranging[2UP,doublesided]
\setuplayout[
width=90mm,
leftedge=0mm,
leftmargin=0mm,
backspace=15mm,
rightmargindistaince=3mm,
rightmargin=25mm,
]
\showframe
\starttext
\dorecurse{4}{\input ward \page}
\stoptext