The difference theory.

Three armies are fighting a war. X army, Y army, and Z army. X and Y are attacking the larger Z army. X army uses spies and assassins to kill off Z armies politicians and nobles, while Y army goes head on into battle with Z army's military. In the end, X and Y army defeat the larger Z army.

 Which army, X or Y, caused the successes and triumph during the war?

Some could say it was X army, for eliminating the people in charge of the military and thus rendering it less effective, whilst also putting the government of army Z into turmoil.

  It could also be said that Y army was the main reason for the successful end to the war. As they greatly weakened the military strength of army Z, thus allowing raids and conquering of cities and towns, then eventually the Capitol of Z army to become easier.

  Though, anyone with a head for strategy could see that it was both X and Y that allowed the best possible outcome and the defeat of the larger Z army. By attacking both political and military adversaries, they sustained minimal damage in return, and also laid the ground for each other to do their own work. 

In summary, if two people were to work towards the same goal, in exact opposite fashions, neither one nor the other could be said to be the more successful of the two, so long as the objectives they set have been accomplished.