Recently I had a PDF file containing multiple pages in A5 format. As I am not always sure of the driver of my printer, instead of asking it to merge two pages on 1 for printing, I wanted to build the A4 myself before sending it to the printer. After some failed tried using "print to file" which didn't work correctly because of the margins.
I finally managed to produce the file I wanted by using the following command :
pdfjam source.pdf --nup 2x1 --landscape --outfile output.pdf
It works perfecly, I have a landscape A4 pdf containing two pages from my source file on each page!