Hello Pier,
I didn't know most of the commands you've used
But according to the manpage for ps2ps, this tool simplifies the postscript-file. I think that converting two files into one (two pages on one sheet) make a complicated postscript code which is not correctly read by the printerdriver. This is corrected by ps2ps.
But I think if you really want a better answer, you should post this issue in another part of forum like the software-forum
http://www.linuxquestions.org/questi...ux-software-2/
Be aware that this issue isn't actually Gentoo-related.
Markus