I would make the merge source doc with linked content controls containing each of the address merge fields. The source doc would include a section for the envelope. Then the merge would need to produce separate documents for each client.

This would allow the address in any of the client output docs to be updated from a single change.

EDIT: Looking at your last response, if the output file is a single document, you could use a styleref field for the second address instance if the document structure was letter/envelope/letter/envelope rather than letter/letter/envelope/envelope

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Andrew Lockton
Chrysalis Design, Melbourne Australia