Dear sir reads as an Indian english address to me. I don't know whether it would be the preferred address in an Indian academia context, but it sounds a little odd in an international context.
If you want to be formal and are writing to US or UK people, then the form of address is "highest title - lastname". This could take the form of "Dear Prof Smith and Dr Doe". In the US, everyone from assistant professor upwards is addressed as "Prof", whereas lecturers, senior lecturers and readers at UK universities are addressed at "Dr".
If you are addressing people in Germany or Austria, the formal way is to list all titles + last name. So here the typical form is "Dear Prof Dr Schmitt". If you were to write in German, you'd add in a "Herr" or "Frau" as well, so "Sehr geehrte Frau Prof Dr Schmitt".
The many countries not mentioned here will also have their own customs. However, given that academia is a very international endeavour, and given the US cultural dominance in international stuff, no reasonable person will be offended by a "Dear Prof X"/"Dear Dr X" address.