I have two pdf files (one is headed notepaper, one is a letter) and I want to overlay them into a single PDF file.

The headed notepaper is one page, the letter is multipage. I only want to overlay the first page of the letter with the headed notepaper.

My only idea is to convert them both to postscript and try to splice the code together, but so far I've not had a very clean experience there.

P.S. this must be automated, and must use Linux.