I did this for a class once. Set your paper size to the size of the cards
you are using. (file-page setup). Make each even numbered page the front and
the odd the back. Then print only the even pages (file-print, then print
drop down box in the lower left has the options for printing only even or odd
pages). Re-insert the cards so the blank side will be printed on and print
the odd pages. Good luck.