You can assume that this letter is addressed to an unmarried couple, as there is not an “and” in-between their names.
There are umpteen etiquette rules on how to appropriately address various titles on envelopes, especially for wedding invitations. If you are simply addressing correspondence or informal invitations, below are some courtesies for you to consider.
Unmarried Couples:
When addressing an envelope to an unmarried couple who lives together, the secret cue they are not married is to write their names independently on two lines and without the word “and,” as in the example below.
For example:
Ms. Holly K. Eason
Mr. R. Stuart Holden
If you’re sending invitation to a couple living together as roommates, but not romantically involved, they should each receive their own invitation!
For Married Couples:
For a married couple, their names are joined together with “and,” and can be addressed on the same line.
For example:
Mr. and Mrs. R. Stuart Holden
If the married couple has different last names, they can still appear on the same line.
When writing the names on two separate lines, their names are still joined with the word "and," in between.
For example:
Ms. Holly K. Eason
and
Mr. R. Stuart Holden
For both unmarried and married couples, the person with the highest rank is always listed first. For same-sex couples, the names may be listed alphabetically.