how do I print two A4 pages on one A3 page?

This article is entirely irrelevant to the question asked, which was about how to print two half-page flyers on a single page.

The margins will not need to be adjusted if the content has been formatted for A4 to begin with. You're correct about the orientation, however: when you select "Book fold," Word automatically changes the orientation from Portrait to Landscape, but this is not the case with "2 pages per sheet." There is more information on this at http://wordfaqs.mvps.org/2PagesPerSheet.htm.

As for duplexing, what you have said is true for any landscape page, not just ones on which you are printing two portrait pages.

Suzanne,  the margins will definitely need to be amended, as when the page layout is changed from portrait to landscape, the margins got automatically changed, with T & B margins changed to outside & inside margins, and vice-versa.  In order to keep the contents on the correct pages, you have to manually amend the margins, so that the correct T, B, outside and inside margins in the landscape layout remains the same as the original portrait page.  Please try this out yourself to verify.

Assuming your printer can print to A3, select that paper size on the Paper tab of Page Setup. Then, on the Margins tab of Page Setup, under "Multiple pages," choose "2 pages per sheet." This will put two A4 pages on the A3 sheet, but you will see them just the same as before (you won't see them side by side). To print two, you can either copy/paste the content of the first page onto the second page or just enter "1,1" into the Pages box in the Print dialog.

Your advice on setting the printer setup is good. 

However, you missed some steps:  In addition to your two steps, set layout to 'landscape' and for printing both sides, set 'flip along short edge'.  Also, the margins will need to be adjustedor otherwise the printed contents will not be exactly the same as original (ie either got more words on the page, or less words).

You may refer the link below, might be helpful

http://office.microsoft.com/en-us/word-help/rotate-page-orientation-between-portrait-and-landscape-in-word-2007-HP001229586.aspx