You need to get your data source into an Excel spreadsheet and then add
a column of sequential numbers, and in the adjoining column enter the
following formula (which in this case assumes that the sequential
numbers are in column A)
=IF(MOD(A1,2)=1,A1+1,A1-1)
When you copy that down, you will have the following two columns of numbers
1 2
2 1
3 4
4 3
5 6
6 5
7 8
8 7
etc.
You need to use copy>paste special values, to convert the formula to values,
then sort by the second column and use the spreadsheet in that format to
merge the second side.

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Hope this helps.
Please reply to the newsgroup unless you wish to avail yourself of my
services on a paid consulting basis.
Doug Robbins - Word MVP
>I have a postcard template and need to mail merge the addresses on the
>front
> and and individualized message on the back. We cannot separate the
> postcards
> and print them individually; they have to remain 8 1/2 x 11.