How to address a letter to a married couple with Jr suffix?

The proper way to address a letter to a married couple with a suffix at the end of the name is to adopt the traditional rule of keeping the man's first and last name together.

Mary and John Smith

This is the proper way to address a married couple when using this format. So it follows that:

Mary and John Smith Jr.

Mary and John Smith III

Mary and John Smith, Esq.

would also be the proper outline.