OK. It's not as if the other detector is sending the gravity wave and the other is receiving it, but that both are seeing an event that happens somewhere else. If the source of the event were equally far from both detectors, the time lag between them would be 0.
The arrival time between the detectors is used to calculate the direction, in the same way your ears determine the direction a sound is coming from. Of course, with only 2 detectors, this will not localize the direction perfectly, but it's a start.