Each Label or enveolpe only pulls from a single contact record. You would have to add a field to the database for them together or use the contact field in one of them. Usually you would think of hte contact field as the outside of the envelope (you could put here Mr. and Mrs, Tom and Mary Smithers) and then use the salutation field for the more personal inside of the lettrer (Dear Tom and Mary or Dear Mom and Dad).
This short article may help with understanding the concept.
