This is not the first time this has happened. United protested some America West/Mesa applications last year for service from LAX
The problem is as stated by United - since only two US ailrines can serve any city pair between the US and Mexico, one airline can effectively control the market if its express airline gets the authority as well.
Obviously, this is extremely anti-competetive.
The DOT is aware of the problem but says that - at the moment - there isn't a lot they can do. It is perfectly legal within the bilateral, since the express provider operates under a sperate AOC
Thus - theoretically - two airlines are serving the route.
Theoretically. I assume it is one of the things being discussed in the present ongoing talks with Mexico about the bilateral.