"To Mr. and Dr. Smith."
Or just separate the names, "To Mr. John Smith and Dr. Mary Smith."
If it is a casual letter you could simply write "To The Smiths"
I am guessing the original form of address you want to modify is "To Dr. and Mrs. John A Smith." But, this is an increasingly archaic form of address and is not applicable in the case of the wife being the doctor or professor nor can it be used when the wife has not taken her husband's name.
When addressing an envelope to the wife of a doctor who is not a doctor herself then and he is Dr. John Doe, then address the envelope to her as Mrs. John Doe or Ms. (Mrs.) Jane Doe.
You address the envelope to the person and only put "X, Y and guest" on the invitation.
Open by Addressee only
Yes, only if you want it to get to the same entity/person as is on the inside.
On an informal letter, you do not usually include the address on the letter itself, only on the outside of the envelope.
You can address it to only one, or address it to both using both names. Either way is acceptable.
If you are not sure they are separated or divorced then address your envelope to both of the. If you know for sure they are separated then send a separate envelope to each one.
In the UK, it is not only the weight, but also the size and thickness of the envelope or package, and the destination (UK or abroad). It is best take take the envelope to a post office to ensure the correct postage is used.
* The address and date go on the invitation only. The little card should only have '_______________and ______ guest. When answered it should appear as Mr & Mrs. John Doe will be attending (and if single) Mr. (or Ms.) John/Jane Doe and guest will be attending. If a guest is coming alone they will only put their own name and no other. On the envelope that goes with this card (to be returned to you) your address should be on it and it should have a stamp placed on the envelope.
Because if you are sending a letter to a person named Jimmy Slim, there could be lot's of Jimmy Slim's, but hopefully only one in the address you have put on the envelope.
In the UK, it is not only the weight, but also the size and thickness of the envelope or package, and the destination (UK or abroad). It is best take take the envelope to a post office to ensure the correct postage is used.
you do not make this yourself , only your doctor can write prescriptions.