What is the word for words that are 'two sides of the same coin'?

What is the word for words that are ‘two sides of the same coin’ as they are not always opposites… such as heads and tails.

Complement?

An example would be ball and strike in baseball. They are the only two possibilities for a pitch not hit. Are the contexts?

Tragedy and comedy but not positive and negative.
Reading and writing and Peace and justice but not winning and losing.