You could address it to Reverend and Reverend Smith. The alternative would be the Reverends Smith.
Dear Mr. John and Rev. Sally Smith,
The proper salutation for a letter to someone with an MDiv degree would be "Reverend" if they are ordained as a minister. If they are not ordained, you can address them as "Mr." or "Ms." followed by their last name.
Pastor is entirely proper, so is reverend. Since they usually have Doctor of Divinity degrees, Doctor is often a preferred title, especially in the more evangelical churches.
If you do not know the appropriate mode of address for his or her denomination (pastor, reverend, brother, father, etc.) then "Doctor" would be appropriate. Usually "Reverend Doctor" is the formal address, as it is less misleading regarding profession.
Dear Father, Dear Reverend Father or Most Reverend Father
This could be done in several ways. I believe the current best practice would be The Reverend and Mr. Smith.
The proper salutation for a probate judge is "The Honorable [Judge's Full Name]."
What is the proper salutation for a register of wills, e.g. Honorable John White?
The proper salutation when writing to a judge is "Dear Judge [Last Name]" or "Your Honor."
What is the proper etiquette to addressing a Reverend in a letter?
the correct salutation for two men is Messrs