Even without stacking the trains I doubt it would make to much of a difference anyways maybe about an extra 72 people per hour if that much. Its not the crew that makes it goes slow, sometimes guest are slow getting off/ on, or left something in the train and have to look for it, or people not riding trying to wait in the unload side of the station. It all adds up, and a few seconds here and there make a big difference. Some times when i was working SOB that would be the case and we would have a train sitting on the brake run while were still loading the train, although i gess you can also see that as we got the last train out real quick. (Although 90 percent of the time we could usually get the train dispatched as the other one was clearing block brakes on a typical cycle).
As "Coasterguy107" said 2 trains is definetly effecient.