If you have Word 2000 or above, by far the easiest way to do this is to use
the "2 pages per sheet" option in the Page Setup dialog (not the one in the
Print dialog). This allows you to treat each half of the page as an actual
page, which allows you to use page borders, page numbering, columns,
margins, etc., just as if it were a real page. In this instance, you'll
still need to duplicate the content on the two pages, but setup is still
easier.
If you have an earlier version, you'll need to divide the page into two
columns; set the space between columns equal to the sum of the left and
right margins. At the end of the first invitation, insert a column break
(Ctrl+Shift+Enter) and duplicate the content in the second column.
