Assuming Word 2000 or above, select "2 pages per sheet" on the Margins tab
of Page Setup (under Multiple Pages). Use Portrait layout if you want two
landscape invitations, one above the other, Landscape if you want two
portrait invitations side by side. You can set up a single invitation and
type "1, 1" in the Pages box in the Print dialog, or you can duplicate the
invitation on "page 2." In either case, you'll need to print 12 copies if
you want them all identical. If they are to be personalized, you'll need to
set up a mail merge.