ORIGINAL: kahnen
I believe the erevo motors have 15* advanced timing. So just running one in reverse, it would be slower than the other. Hopefully you just have the motors mixed up. If not you may have got two with advanced timing.
Here is a thread that supports my thoughts on the timing. Its funny cause Traxxas official says it doesn't but the motor part number shows it does.
http://monster.traxxas.com/showthread.php?t=433140
If you look at the motor positions in the Erevo you can see the motors run the same direction, that the reason they have the timing in those motors.
The e-revo needs one reverse motor with its setup, the one in the front of the photo is a reverse model. If both motors were in the same side of the spur they would turn the same way.