You can address a letter however you want, but when you're addressing invitations or formal business correspondence, it's important to follow the rules of social etiquette. Usually they are pretty easy to figure out, but sometimes tricky situations come up. For instance, many people don't know how to address a letter to two doctors in the appropriate professional fashion. It's very easy once you know how to do it, and the doctors will surely appreciate your efforts.
Do a little research. Most likely you're writing to two doctors because they're married to one another, but just because they're married doesn't mean they share a last name. Also, just because they have the same name doesn't mean they're married. They could be father and daughter, sister and brother, or otherwise related. All of these things will affect how you address the letter. If you don't feel comfortable asking the doctors themselves, call one of the doctors' offices and explain your situation to the receptionist.
Head your letter. Write the date in the top right corner. Then skip two lines and add the doctors' names on the left. If the doctors are single or they're married to each other but don't share a last name, list their names one above the other, writing them as "Dr. First Name Last Name." If they're married to each other and do share a last name, write "Drs. First Name and First Name Last Name" listing the man's name first, both on the same line. (If they are same-sex partners, just choose a name to list first.)
Skip two more lines, then write your greeting. For doctors who are single or don't share a last name, write "Dear Dr. Last Name and Dr. Last Name." For doctors married to one another and who share a last name, write "Dear Drs. Last Name." It may look a little funny, but it is correct.
Address your envelope using the same conventions you used in Step 2. If you have space constraints, it's OK to go with the rules in Step 3, although it is not considered the most correct method.
To be even more formal, write out the word "Doctor" instead of using the abbreviation. If you're writing to a married couple in which only one member is a doctor, address the person who isn't a doctor as "Mr.," "Mrs." or "Miss" as applicable. For example "Dr. and Mrs. Smith." Remember that the doctor title applies not only to medical doctors, but also to people with Ph.D. degrees. Always type professional correspondence or write it neatly using black or blue pen.