What do despotism and theocracy have in common?

In the definition, few things.

Despotism is a form of government in which the powers is centralized in the hand of one man. Despotism is different from dictature in the sense that, in a dictature, an individual is entrusted all political powers.

Theocracy is a form of government in which the religious power controls the civil life.

You can say some theocracies are a form of "religious despotism".