I just came across a nice tool on superuser.com called PDFjam that can do all of the above in a single command:
pdfjam --nup 2x1 file1.pdf file2.pdf --outfile DONESKI.pdf
It has other standard features like page size plus a nice syntax for more sophisticated collations of pages (the tricky page re-ordering necessary for true booklet-style page imposition).
It's built on top of TeX which is, whatever it is. Installing is a breeze on Ubuntu: you can just apt-get install pdfjam. On Mac OS, I recommend getting BasicTeX (google "mactex basictex"; SO thinks I'm a spammer and won't let me post the link).
This is a lot easier and more maintanable than installing both pdftk and Multivalent (on both Mac OS for dev and Ubuntu for deploy), which wasn't going so well for me anyway...!
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
