You must take the Knowledge exam first, then the lab.

This link gives you the topics covered:

http://www.microsoft.com/learning/en/us/certification/master-sql-path.aspx

I would assume the Knowledge exam incorporates subjects from both the development and DBA exams, but you will need to know the subjects much more in-depth.

I'm not sure what the delay is between passing the Knowledge exam and taking the Lab exam - maybe somebody else can help?

Mike.