I believe this is an issue that has been reported before using "Book fold"
or "2 pages per sheet." I'm not sure we ever established what causes it, but
you might try switching to a different printer driver to see if you still
see these lines in Print Preview. If you're not seeing them in Print
Preview, then likely there is some setting in the printer Properties, though
I can't think what it might be.
--
Suzanne S. Barnhill
Microsoft MVP (Word)
Words into Type
Fairhope, Alabama USA
http://word.mvps.org