Hi
Usually when its a 2 part exam you can sit them at different times allowing to study and pass one and then concentrate on the other. Only when you have the two exams passed will you be awarded the certification. Also it doesnt matter what order you do them in, as long as you have the combination of both.
Just as a matter of note, you need 4 exams for the Dynamics AX Developer cert. The 2 you have listed are the core (IE must be done) and there are 2 electives where you have a choice of 2 exams you want to do.
MCP,MCAD,MCSD.NET,MCTS*7,MCPD*3,MCITP*2,MCDBA
Blog:
http://certsandprogs.blogspot.com