The third possibility is that one of the incompatible outcomes occurs and the other does not. The two alleged supreme wills would then stand in differing positions: one has prevailed and the other has not.
If the second agent’s intended result is genuinely prevented by something beyond his power, his incapacity or lack of independence comes into question. A being supposed to be independent in essence and unlimited in power cannot, under that description, be genuinely overruled.
This is the pressure point of the argument: two allegedly sovereign, all-powerful agents cannot straightforwardly be reconciled with one rendering the other ineffective.