# [NTG-context] Duplexing a single-sided layout?

I have a document in which left and right cannot be equated with inner and
outer respectively (outside the header texts). So I use
\setuppagenumbering[alternative={singlesided,doublesided}]. But I still need a
gutter along (what would be) the inner margin. So, how would I go about
duplexing a single-sided layout?

For example:

\definepapersize[custom][width=335mm,height=500mm]
\setuppapersize[custom][B3]
\setuplayout[%
width=middle,%
height=middle,%
backspace=90mm,%
topspace=25mm,%
leftmargin=30mm,%
rightmargin=15mm,%
leftmargindistance=5mm,%
rightmargindistance=20mm,%
location={doublesided,duplex},%
bottomspace=25mm,%
cutspace=90mm]
\setuppagenumbering[alternative={singlesided,doublesided}]

Sincerely yours,