If there are multiple effects that attempt to modify how a creature assigns combat damage, that creature’s controller chooses which effect applies. For example, if Narod, the Beige Flower and Doran, the Siege Tower (“Each creature assigns combat damage equal to its toughness rather than its power”) are both on the battlefield, each time a creature would deal combat damage, that creature’s controller chooses whether it assigns combat damage equal to its mana value or its toughness.