MEX is a bit of a weird airport - all international arrivals and departures mix witht he exception of those from Colombia and certain other "select destinations." If you have only carry-ons or bags are checked through, you can literally walk off one aircraft, wander the shops (I love the "Pineda Covalín*" shops with the autochthonous designs in textiles, jewelry and things like ties - wonderfully designed, authentic and a source for unique gifts from México,) proceed to your departure gate (it can be some distance as the airport is one long skinny building) and board.

Be aware Mexico's version of the TSA stands around and often hawks those who are standing closest to the gate for a secondary inspection.

*The Pineda Covalín site isn't working in English- but you really go there for the pictures.