It is annoying that there is no simple solution for printing two pages on a single sheet of paper under UNIX systems, while under Windows it is simply a select box in print GUI. There are two ways under UNIX to do this, which are mpage and pstops.
Using mpage
$ mpage -2 input.ps > output.ps
Using pstops
$ pstops "2:0L@.7(21cm,0)+1L@.7(21cm,13cm)" input.ps output.ps
Note that files to be printed, but in other formats, need to be converted to PostScript (.ps) format first.