I am wondering how to write email to both a professor and an assistant professor in the same email.
Shall I use "Dear Professor … and …" or "Dear Professor … and Assistant Professor … "?
I have found this existing question, which says that I should use "Dear Professor …" for assistant professors. However, I am wondering if it is appropriate to use "Dear Professor … and …"?
I am in Japan, but writing in English as the assistant professor does not speak Japanese well.
You will be on the safe side if you write "Dear Professors Smith and Miller". They are both professors, and the fact that they have different rank is unimportant in the address. If one is junior to the other, put the senior one's name first.
In general, talk to how others (older) students do that. You have a support network for these sorts of questions around you. Use it!
In the UK, the title 'Professor' has historically been reserved for full professors, with lecturers (assistant professors), senior lecturers (associate professors), and readers generally addressed as Dr. However, the current University of Oxford Style Guide now notes that Associate Professors "may, if they wish, use the title of ‘Professor’, or they may keep their previous title of ‘Dr’. This demonstrates how non-uniform it is! It really does seem to depend on what country you are in, and even what university you're at! But I personally would address full professors as Professor, and everyone else as Dr. (In some places in Europe, though, everyone is called 'Professor', and I've noticed that in some places in North America, professors often prefer their Dr title!)
Assuming you're in the UK:
If you know them: "Hi Bob and Dave".
If you don't know them and want to be formal: "Dear Dr X and Dr Y".
If you don't know them and want to be more casual: "Hi everyone".
If you want to play it safe: "Good afternoon/evening/morning".
Honestly, unless you're writing to someone very stuck-up, nobody is going to care or even notice if you just use their first name. If in doubt you can easily avoid names and titles.
Really depends on the country and even the people you are talking to (in countries like Canada and the US, profs come from all over the place).
Little anecdote: ten or so years ago, I was department head and had to obtain references for short-listed candidates to an assistant prof position (we hadn't started using mathjobs yet). One of the names rang a bell, but I couldn't quite place it. I sent my emails without paying any more attention. "Dear First Name, X has given your name as a reference and I am writing to request a letter". Emails come back with letters and I realised at that point that the reason the name rang a bell is that it was a Fields medalist (with an email from a different uni than his "canonical " one, hence my confusion). I was happy, because in hindsight, I probably would have agonised a long time on how to start my letter. He replied using my first name and didn't seem one bit annoyed that I wasn't more formal.
In short: don't overthink it. Most profs don't really care.
For a first contact, you can always play it safe and use "Professors X and Y" and see what they use in their answer.
More of a comment than an answer, but I think it's worth pointing out a couple of general rules of thumb.
Academics are routinely referred to with a wrong honorific. I'm often addressed as Prof. (which I am not), Dr., Mr., or by first name. There's not a lot of risk of annoying someone this way, we're too used to it. The worst I would realistically expect is a chuckle or a raised eyebrow.
When in doubt, it's always safe to err on the side of being more deferential. If you mistakenly address someone as Prof., they won't mind, especially if they're an assistant professor. It's almost always fine to use Dr. - that's a title that academics will almost universally have, and is a more clear-cut distinction than professor. Most people won't mind using first names, but some might, so it's best to exercise caution.
In the United States, you should never address anyone as "assistant professor" or "associate professor." To do so is simply rude. Anyone whose title contains "professor" gets the courtesy title of Professor; even instructional faculty whose title is something like "instructor" can typically be addressed this way.
Dear colleagues,
That seems intuitive. accurate, appropriate, and warm.
Assuming they are both Doctors, "Dear Dr. A and Dr. B" or "Dear Drs. A & B" should suffice.
