There are many ways to address married couples in a letter, just as there are many different types of married couples to whom you can write. Each couple will have a different preference about how to be addressed, especially in an era when there are many unconventional marriages. Some couples keep their own last names, whereas some share a last name. Keep your recipients' sensitivities and preferences in mind when you address a letter to married people.
Address the letter using both partners' first and last names if the married couple have both chosen to keep their original surnames. Consider placing "Mr." and "Mrs." in front of both of the first names. An example would be "Mr. Tom Edwards and Mrs. Lisa Thomas." Or, if you choose or the couple prefers, simply "Tom Edwards and Lisa Thomas."
Address the letter with a "Mr." and Ms." if the wife so prefers: "Mr. Tom Edwards and Ms. Lisa Thomas."
Address the married couple by the same last name, and use only one first name; e.g., "Mr. and Mrs. Tom Edwards." This style is perfectly fine if you are certain the wife has chosen to take her husband's last name in place of hers.
Address a gay couple by adding the "Mr." or "Ms." before both of their full names. An Example of this would be "Mr. Timothy Dew and Mr. Paul Voisin," or "Ms. Teresa Thomas and Ms. Sabrina Edwards." If addressing an inside envelope of an invitation, use only their titles and last names.
Address a Rabbi and his wife by writing his(or her) title, name, and then the spouse's name, using the "Mr.," "Mrs.," or "Ms." format before their full name. An example of this would be " Rabbi Tom Edwards and Mrs. Lisa Edwards."
Address a physician or doctor by stating their particular title before their name, usually "Dr." or "Md.," and then the spouse's with the "Mr.," "Mrs.," or "Ms." format, then their full name. An example of this would be "Dr. Edward Thomas and Mrs. Lisa Thomas." If they are both physicians, place the title before both of their names.