>>42259085
I further headcanon Celestia and Luna had to use the Elements together because each of them activated complementary Elements. When Luna became Nightmare Moon, Celestia couldn't beat her at first because the Elements wouldn't cooperate. Nightmare Moon was able to rule Equestria for a while until Celestia grew as a pony and learned how to use the Elements properly and by herself, and finally succeeded at banishing Luna to the moon.