If you have Word 2000 or above, your best bet is to set it up this way from
the outset, using the "2 pages per sheet" option in Page Setup (not the
Print dialog). You'll have to create your flyer twice, but that's fairly
easy with Copy/Paste.

If you have Word 97 or earlier, you won't have this option, but you can
divide the page into two columns and accomplish much the same thing. Note
that you will need to set the distance between columns to the sum of your
left and right margins (which should be equal).

If you create the flyer as A4, you can (in Word 2000 and above) print "2
pages per sheet" from the Print dialog, but note that if you have just one
page, you need to enter "1,1" in the Pages box.