I may have a partial answer. (I developed this in Word 2007 on Windows 7; your YMMV.)
I don’t have access to a PDF writer; maybe the following techniques will work on yours.
In Word, set the paper size to 6”×8”, and set it to print two pages per sheet. This sounds like what you have already tried, but I believe you’ve been doing it in the Print dialog; I did it in Word.
Print to the “Microsoft Office Document Image Writer”. This is a virtual printer that acts like printing to a PDF file, except it prints to a TIFF file. AFAIK this comes with Microsoft Office.
Now, in the Print dialog, go to Properties and set the page size to 6”×8”.
Print (and identify an output file).
I got a multi-page TIFF file, where each page is 1200×1600 (pixels), and the resolution is 200 dpi — i.e., it is 6”×8”. Each page of the TIFF file is two pages from the Word document, each 6”×4”, and stacked (vertically).
The image below is what the first page looks like:
I believe this is what you want, or at least close to it.
The catch is: I haven’t been able to figure out how to print this properly. Oh, printing the first page isn’t too hard. But not many programs understand multi-page TIFF files (like animated GIFs), so they can access and display only the first page. Windows Photo Viewer is the only program I was able to find that can handle the multiple pages.
But when I used it to print the file, it insisted on up scaling the image to fill the 8½”×11” page — and I know that’s not what you want.
Try setting the paper/page size to 6”×8” in the PDF writer properties, and see if you can produce a PDF file that you can print, or
Do what I did, and try to find a tool that will print multi-page TIFF files without changing their size.
P.S. I tried to upload the the complete TIFF file to imgur.com, and it converted it to PNG during the upload — keeping only the last page!