I'm not sure if I should address them individually or as a pair
Sehr geehrter Herr und Frau Schmidt or Sehr geehrte Pfarrer Schmidt
The most common way to do this is
Sehr geehrte Frau Schmidt,
sehr geehrter Herr Schmidt,
If you don't want to specifically adress them in their capacity as pastors, I would leave the "Pfarrer" out.