In a formal emai/business letter, if you are addressing it to two Deans, is the proper format:
Dear Dean Jones and Dean Smith:
Dear Deans Jones and Smith:
(**NOT an expert on this topic...)
... All I know is that the first one sounds more respectful of their titles.
The second one ("Deans") even sounds sort of pretentious to me, but again, I'm no expert on business letters!