Dear John and Jane,
Dear Mr Smith and Ms Doe-Smith, [this is the proper answer in the "book"]
or ... and this I insist is still correct because I simply don't subscribe to that hypenated nonsense:
Dear Mr and Mrs Smith,
If you remember nothing else, please remember that Ms is not an abbreviation and therefore NEVER requires a period after it. You may, or may not, as your preference indicates, put a period after Mr or Mrs (that depends on the continent you are addressing), but Ms is not an abbreviation and to put a period after it is simply ignorant of that fact. It's a "symbol" likg kg or ml or the like.
- For Teachers