How do you sign a letter from a married couple?

Answer 1

Depending on the degree of formality in the letter it can be Mr. and Mrs. Smith, or John and Jane Smith.

Of course if the couple retained their pre-married names, you would use John Smith and Jane Doe (or Jane Doe and John Smith) ;)

Answer 2

The person who penned the letter. In the case the letter is of a personal &/or informal nature the person writing it my sign the other spouse's first name before their own.

Answer 3

BY using their names such as: Carrie and Russell Moran.