How can one act relate to two powers? The answer keeps one act as the object while distinguishing two ways of relating to it. A person's writing is one occurrence, but creating the act and performing it voluntarily are not the same sort of relation.
Competition between independent creative causes would arise only if the two powers were asserted to originate the same act in precisely the same independent sense. Kasb, however, concerns the human agent's voluntary involvement and accountability.
Using the word power in both contexts does not make the two powers equally independent. Divine power is uncreated and independent; human capacity is granted and limited.