Divide Landscape into 2 pages

1. On the Page Layout tab, in the Page Setup group, click the dialog launcher (the small arrow in the bottom right corner). This opens the Page Setup dialog. An easier way to open the dialog, if you have the horizontal ruler displayed, is to double-click at the top of the ruler.
2. On the Margins tab of Page Setup, beside "Multiple pages," click the dropdown and choose "2 pages per sheet" as shown below.
3. With the page orientation set to Landscape, you will have two A5 pages on an A4 sheet. These will behave just like full pages in every way; you can add page numbers, page borders, etc., just as if they were separate pages. Unless you choose to display two pages, you'll see only one page at a time.
4. If you are creating a booklet, you may prefer to select "Book fold." This has the same effect, and looks just the same when you are editing the document, but when you print, Word will print the pages in the correct order to be folded into a booklet.

Edit: This process has now been detailed at http://wordfaqs.mvps.org/2PagesPerSheet.htm.