Which incidents did not result in accidents because of the actions of the second pilot on board?
During the 2005 Logan Airport runway incursion, the First Officer noticed the other plane and realized that they could collide. Telling the Captain to "keep it down", he pushed the control column forward and passed under the other aircraft as it took off. 

During United Airlines Flight 232, the actions of the flight instructor for the type, Dennis E. Fitch, brought in additional expertise that significantly contributed to success. E. Fitch joined the crew during the flight as an additional pilot.

On the day before Lion Air Flight 610 crash, that plane, already with problems, has been saved by off-duty pilot who correctly identified the problem (and suggested to disable the flight control system).