If two equally sovereign creator gods disagree on any point, noncontradiction is no longer a valid test of truth. Without noncontradiction, there can be no test of truth at all, and rational thought goes out the window.
But the problem is worse than that.
Since the gods are equally sovereign, neither is in control of the other one. This means that neither one can guarantee the behavior of the other one. This means that even if they agreed on every point all the time, neither one of them would ever be able to guarantee their agreement.
So why is this a problem?
This is a problem because humans come pre-programmed with the belief, the first principle, the presupposition, that contradictions are always false. But which one of these gods pre-programmed us humans to believe that all things are non-contradictory? God A, who doesn't know whether God B will contradict him? Or God B, who doesn't know if God A will contradict him? Either way, our first principle or initial assumption of universal noncontradiction is rationally unjustified, meaning there is no good reason for us to believe it, even if it is true.
If there is no good reason for us to hold to universal noncontradiction, there is no good reason for us to hold to anything. All of our conclusions are based on the premise of noncontradiction. If we hold to it irrationally, we hold to every belief irrationally.
According to Plato, Socrates saw this problem with respect to moral facts, and his conversation with Euthyphro in Plato's Trial and Death of Socrates provides the basis for the argument against a plurality of ultimate authorities.
Some may wish to use a different definition for polytheism, claiming that only one God created everything, and lesser gods are then subject to Him. I would call this a form of monotheism. Others may say that there are lots of gods, but none of them created the universe. I would call this a form of atheism and it is even easier to prove atheism is false than it is to prove that polytheism is false! Polytheism then stands apart from both monotheism and atheism as having multiple, equally sovereign creator gods.
Under Christianity, there is one God -- one Creator. His identity corresponds directly to His identity, establishing the Law of Identity (Exodus 3:14). He never contradicts Himself, meaning that if He creates something, it is created, establishing the Law of Noncontradiction (2 Timothy 2:13). Our omniscient, omnipotent God can rationally justify our pre-programmed idea of noncontradiction. He knows all of reality is non-contradictory, because He made it! The Christian God Himself makes rational thought possible for His created humans.
As Greg Bahnsen rightly said, the most basic proof of Christianity is that without the Christian God, it would be impossible to prove anything at all.