I'll be the first to say that learning PostScript is no small task; I'm pretty sure you can get around this in cheaper ways.

One way that needs no external tools; What about re-creating the widget(s) making up the header canvas on the data canvas? Place them on the right coordinates for page one, call postscript(), move them to coords for page two, etc.. and just destroy them once you're done printing.

Another possibility is using ImageMagick; you can overlay images using the command-line 'composite' program (may be avaliable through a Perl binding also). This assumes, of course, that you supply a header.ps by either generating it from the header canvas or optionally supplied as a static file from Acrobat or some other tool.
composite header.ps data.ps page.ps
There is also most likely a way to do this using some tool/script in the 'psutils' package http://www.tardis.ed.ac.uk/~ajcd/psutils/. You can use this package to combine several .ps to one file.

Just my two cents. If you post your "dual canvas" code, it's easier to make helpful suggestions :-)