Thanks everyone. While I hadn't thought to consider the cruising speed (or takeoff/descent rates), it doesn't seem commonplace that aircrafts on the same route overtake one another. (The DL flight shows a scheduled departure earlier than UA; with a later scheduled arrival time.) I've certainly not experienced it on, say, SFO-NRT.
Am I at least correct in assuming that both flights typically use the same runway A in departing NRT (the one closer to terminal 1)?