I only know two ways:
a. use a macro to resequence the data - if you search this group for e.g.
"Doug Robbins postcard" (without the quotes) in Google groups you should
find some code by Doug Robbins that will do this for a Word data source.
b. use SET fields to save the values for each card until you need them
again. Unfortunately, this does not wourk with all field types, but for
example, let's assume your data is sequenced correctly for the front side of
the cards and you have a field called myfield.

In a typical one-sided merge you would have:

side 1 row1 column1: (card 1 of 4)

{ MERGEFIELD myfield }

side 1 row1 column2: (card 2 of 4)

{ NEXT }
{ MERGEFIELD myfield }

side 1 row2 column1: (card 3 of 4)

{ NEXT }
{ MERGEFIELD myfield }

side 1 row2 column2: (card 4 of 4)

{ NEXT }
{ MERGEFIELD myfield }

How to modify this depends on whether you can get double-sided printing to
work. To do that, I would try the following:
a. make a page break after the table.
b. copy the contents of the table, paste onto the second page and replace
all the fields with some simple data to test the next two steps (any old
text)
c. go into File|Page Setup and/or your printer driver and do what is
necessary to set up double-sided printing
d. use some ordinary paper and a small amount of test data to test that
double-sided printing works at all, and particularly after the first two
sides

If that works, l then alter page one as follows - there are various ways to
do this but all the {} need to be the special field quotes you can enter
using ctrl-F9.

side 1 row1 column1: (card 1 of 4)

{ SET pc1myfield "{ MERGEFIELD myfield }" }
{ MERGEFIELD myfield }

side 1 row1 column2: (card 2 of 4)

{ NEXT }
{ SET pc2myfield "{ MERGEFIELD myfield }" }
{ MERGEFIELD myfield }

side 1 row2 column1: (card 3 of 4)

{ NEXT }
{ SET pc3myfield "{ MERGEFIELD myfield }" }
{ MERGEFIELD myfield }

side 1 row2 column2: (card 4 of 4)

{ NEXT }
{ SET pc4myfield "{ MERGEFIELD myfield }" }
{ MERGEFIELD myfield }

Then set up page two as follows:

side 2 row1 column1: (card 2 of 4)

{ REF pc2myfield  }

side 2 row1 column2: (card 1 of 4)

{ REF pc1myfield  }

side 1 row2 column1: (card 4 of 4)

{ REF pc4myfield }

side 1 row2 column2: (card 3 of 4)

{ REF pc3myfield }

Peter Jamieson