[texhax] Splitting 2-up pages

[Joel C. Salomon]

(Cross-posted from <https://tex.stackexchange.com/q/476431/2966>.)

I have a PDF document “printed” 2-up, so that on each 11″×8½″ page there
are two 5½″×8½″ pages I’d like to extract. How do I go about this?

I can do this manually by writing

\documentclass{minimal}
\usepackage[papersize={5.5in,8.5in}, margin=0pt, ignoreall]{geometry}
\setlength{\parindent}{0pt}
\usepackage{graphicx}
\begin{document}
\includegraphics[viewport= 0 0 396 612, page=1]{two-up.pdf}
\includegraphics[viewport= 396 0 792 612, page=1]{two-up.pdf}
\includegraphics[viewport= 0 0 396 612, page=2]{two-up.pdf}
\includegraphics[viewport= 396 0 792 612, page=2]{two-up.pdf}
% etc.
\end{document}

but this is repetitive and error-prone. Is there a way to do this
automatically for the entire book?

—Joel C. Salomon