You need to use a label type type mailmerge main document (labels are
actually table with fixed cell dimensions) the layout of which suits the
stock that you are using to create the cards.  As a result of the labels
being acutally cells, you can rotate the text (mergefields) in the cells by
plus or minus ninety degrees so that when you fold the card, you have the
text the right way up.  Each card will need to consist of two cells and you
will need a <<Next Record>> field before the first merge field in each "tent
card"(pair of cells)in the mailmerge main document OTHER THAN the first set
on the sheet.

Also, take a look at the article "Mailmerge Labels with Word XP" on fellow
MVP Graham Mayor's website at:

http://www.gmayor.com/mail_merge_labels_with_word_xp.htm