I have a doubled-sided A5 landscape document with the following page order:

As my printer supports only A4 paper, I want to print two of these pages on one portrait A4 paper so that I can cut it in half to get sheets of the original size. In other words, what I need is the following page layout:

(The line separating the two pages is for demonstration purposes only, it is not part of the final result.)
How can I achieve this layout with pdfpages?
The "naive" approach
\documentclass[a4paper]{article}
\usepackage{pdfpages}
\begin{document}
\includepdf[pages=-,nup=1x2]{a5-document}
\end{document}
yields

so the front and back side of each page are one the same side of the paper, instead of appearing on opposite faces.
At the moment, I'm using the following manual solution, which is quite tedious and error-prone, especially for large documents:
\documentclass[a4paper]{article}
\usepackage{pdfpages}
\begin{document}
\includepdf[pages={1,3,2,4,5,7,6,8,...},nup=1x2]{a5-document}
\end{document}

pdftkin combination with a small script is also possible. – Marco Daniel Nov 29 '11 at 18:46\includepdf[pages=-,signature=4,landscape]{doc}what you're looking for? – egreg Nov 29 '11 at 18:47pdftksolution would also be acceptable. – diabonas Nov 29 '11 at 19:22\includepdf[pages=-,signature=4,landscape]{doc}, the pages are placed at the correct position. They are, however, shrunk and rotated by 90 degrees (see the attached image). – diabonas Nov 29 '11 at 19:29\includepdf[pages=-,signature=4]{doc}. I guess it'll work.:)– Paulo Cereda Nov 29 '11 at 19:42