I couldn't get templates to print correctly either, but I finally figured it
out.
Design your postcard. Do your merge. Your postcard should be two pages. One
page with the design and the other page the mailing information. The key is
in the print set-up, not the merge.
Select: "Print Merge."
Select: "print multiple pages per sheet"
Paper Size: "letter"
Orientation: "Landscape"
2-Sided Printing Options: "two-sided, flip short side"
The result is an 8 1/2 x 11 paper with two post card designs along the left
and two DIFFERENT addresses (a,b) along the right on paper side. If you flip
the paper over, like turning a page in a book, you have postcard designs
along the left again and two more different address (b,c) along the right.
When you cut them, you get 4 postcards with the same design on one side and 4
different address (a, b, c, d) on the other.
Finally success!
Shelley