I would do exactly the same, only I would print the header page first, then return the paper to the printer and print the main letter. Obviously the main letter would have to start some way down from the top of the page otherwise it will overwrite the header.
Or am I missing something in your post?
Is it possible to do the following in Word?
I have my headed paper on word so when I create a letter, I print the letter then put the paper back in the printer and print the headed paper over the top of it. I dont want the headed paper permanently on the letter as I print several copies for other files and they do not need to be on headed paper. I used to use a word processing programme that did have this facility and it was called overprinting. Can anyone point me in the right direction?