Printing a mail merge in Word 2007/2003

We use headed paper for our letters and the first page is pre-printed differently from the following pages. The HP printer we use is pre-programmed to print Page 1 from the Top Tray and Page 2 onwards from the 2nd Tray. This does not work when we create letters using Mail Merge. The first page of the merged document is printed from the Top Tray but all the rest are printed from the bottom tray. The first page of each letter is recognised by Word as being a new page 1. Can anybody tell me how we can get the printer to recognise each new letter as having a Page 1 and needs to have that page printed from the Top Tray? One of our computers uses Word 2007 and one uses Word 2003, the same problem occurs from both of them.