How to address an envelope/head a letter

I need to send a letter to the parents of a young student who is coming to stay with me in July. I want to send it to both parents and in England we would write 'Mr and Mrs XXX' What do you use in Japan? Is it correct to address a letter to both parents?

Jackie,

If you are writing in English, then you should use the standard "Mr and Mrs XXXX". If you are writing in Japanese, then I think it would be normal to write just "〇〇様" (XXXX-sama).

In Japanese language, we would write, actually at the end of the letter；

(surname) (husband's first name)様、(wife's first name)様"

If you're writing the envelope in English, why not simply write, as mentioned, "Mr. & Mrs. (surname)" because that helps the postman.

And then on the letter, write "(surname) (husband's first name) sama, (wife's first name) sama," at the top of the letter. If you don't know their first names, just write "(surname) sama". That sounds natural and friendly. Another idea is to write on the letter, "To the parents of (first name of student)," which is another common way of addressing parents in Japan.

In Japanese, we normally don't use titles that represent gender (such as "mister"), but the wife would probably find it nice to see her presence, since it's often the husband that gets all the attention.