Envelope Addressing

Are their last names different? If not it would look and sound good writing Professor and Professor K.

Do they have doctorates?

My cousins are both PhDs. I address things to them as the Drs. Their Last Bane

Dr and Dr if they have PhDs, or Prof. and Prof.

No doctorates. Both have same last name.

My FH and I have decided when we get our PhDs, we'll prefer the Doctors So-and-So I don't see a problem with saying plural Professors. But do you even need to address them as Professor? I wonder if Mr and Mrs would suffice.

For an envelope, social it would be:

Dr./Mr./Mrs. (Full name)

(Address)

Since you've said that they do not have doctorates, just address the envelope with Mr. & Mrs. and you should be fine.

PhD student here. If they don't have doctorates, they're not professors, traditionally speaking. I would address it as Mr. and Mrs.

Professors Kishimoto should work or Mr and Mrs Kishimoto.