Email to two academics of differing familiarity I am sending an email to two people: Prof FirstName1 LastName1 and Dr FirstName2 LastName2. The former has a PhD and is officially a professor (with tenure, if that matters) with whom I am not intimately familiar. He prefers to go by Prof or Dr LastName (I have no problem with it). The latter I have worked with for a few months now and strongly prefers not to be called Dr (much less Professor, as he isn't one; he does research and lectures but the institute at which he works does not grant that title). Instead, I call him FirstName2 as he has instructed me to do. The two are of a similar age and experience (as far as I know). 

My question is, when emailing them, should I say "Dear Prof LastName1 and FirstName2," or a more formal (but possibly upsetting) "Dear Prof LastName1 and Dr LastName2," or something else altogether? Furthermore, what would be the most polite thing to call each of them in a meeting where both are present? I would like to respect both of their preferences, but not respecting FirstName2's title seems like it could be taken the wrong way (i.e. insulting). 

I ended up going with "Dear all," even though there are only two people in the email but I am wondering what the most polite way would have been (for future reference).
&quot;Dear all&quot; sounds perfectly fine to me, and using plural is an easy way out, especially if not all the recipients have a honorific. &quot;Dear professors/ colleagues/ both&quot; are other options, although in that case one of the recipients would not like the first one. For two recipients only, using both names as @vonbrand suggests is fine but I would avoid it for more than two.