Just remove this line:
MailManager.Instance.SendMailToAccreditor(aua);
You have already added both recipients to newMessage. I don't totally understand what you are doing with your classes, but this will reduce the code to only send one message via Microsoft exchange. I am guessing that MailManager is a internal class that you built and the SendMailToAccreditor method creates a message fully populated email when called with a parameter of type AccreditorUsersAbsences.
I have no idea what you are wanting but if you just want one email to go out remove the line above if you want the email that is sent from MailManager to go out (email body wise) then you will need to modify that class and remove the newMessage.
I hope that helps. Without knowing exactly what you want and Combining them into one email is not a good answer because you are sending 2 messages one is being generated in this code and one is being generated in MailManager so more info is needed before I can give anymore help.
Damien