\unprotect

\installpagearrangement FLASHCARD
  {\dosetuparrangement
     \v_page_target_nx
     \v_page_target_ny
     {\numexpr\v_page_target_ny*2\relax}%
     {\numexpr\v_page_target_nx+\plusone\relax}%
     {\numexpr\v_page_target_ny+\plusone\relax}%
   \pusharrangedpageFLASHCARD\poparrangedpagesAB\relax}

\newcount\arrangedpageshiftX
\newcount\arrangedpageshiftY

\def\dopusharrangedpageFLASHCARD#1%
  {\ifnum\arrangedpageshiftX<\arrangedpageX
     \ifodd\arrangedpageN
       \handlearrangedpageXandY{#1}0\arrangedpageshiftX\arrangedpageshiftY\arrangedpageA
     \else
       \handlearrangedpageXandY{#1}0{\numexpr\v_page_target_nx-\arrangedpageshiftX-1\relax}\arrangedpageshiftY\arrangedpageB
       \global\advance\arrangedpageshiftX\plusone
     \fi
   \fi
   \ifnum\arrangedpageshiftX=\v_page_target_nx
     \global\arrangedpageshiftX\zerocount
     \global\advance\arrangedpageshiftY\plusone
   \fi}

\def\pusharrangedpageFLASHCARD#1%
  {\advancearrangedpageN
   \reportarrangedpage\arrangedpageN
   \ifnum\arrangedpageN=\numexpr\v_page_target_xy*2\relax
     \dopusharrangedpageFLASHCARD{#1}%
     \global\arrangedpageshiftX\zerocount
     \global\arrangedpageshiftY\zerocount
     \poparrangedpages
   \else
     \dopusharrangedpageFLASHCARD{#1}%
   \fi}

\protect

\setuppapersize[A7][A3,landscape]
\setuppaper    [nx=4,ny=2]
\setuparranging[FLASHCARD]
\setuplayout   [location=middle,marking=on]

\setupbackgrounds[page][frame=on]

\starttext

\dorecurse{20}
  {\startstandardmakeup[align=middle]
   Front side \recurselevel
   \stopstandardmakeup
   \startstandardmakeup[align=middle]
   Back side \recurselevel
   \stopstandardmakeup}

\stoptext