As first author, what should be included in the first formal communication [email] with other coathours I am in the process of writing a paper ( public health/ policy) for submission collaborating with two professors. Both are supportive and interested in my project. 

separate meeting with both were held and now I would like to send a formal email to both of them. what should such email include at this stage? giving that we have an  agreement on a general deadline and the targeted journal. 

Thanks!
Just don't sweat it.  They are colleagues just be friendly and share the info you want to share.

I get the sense that you are not a professor.  Maybe you're still a grad student.  I remember feeling like this but the professors are (hopefully) looking at you as kind of colleague in the making not an undergrad who needs to make some show of respecting their authority and position.  Just be friendly and make sure you communicate everything they need to know and, if you are unsure if something is ok say so and ask, e.g., don't try and dictate the author order.

Of course, I'm a mathematician so I'm not in public health but if you are talking about United States academic culture and you're a genuine part of that (e.g. grad student) don't sweat the formality so much.  Of course, cultures differ so your mileage may vary if you are working with foreign professors who weren't educated in the US.