I think you’re being too technical in your reading of the treason clause. When the Constitution refers to the “United States” and “their enemies,” that does not mean that two states agreeing that a group of people are invaders means that those people are “their enemies,” and that any support for those people is treason. The “United States” does not mean two or more states that disagree, it means the Union.
Think about the logical extension of your argument. Your argument essentially gives two people (the governors of two states) the power to declare war. That is contrary to Article I’s provision granting Congress the power to declare war, raise armies, repel invasions, etc.
No, the power to declare War is reserved to the Congress; however, when States are being invade a state of war already exists (invasion being an act of war, obviously). Just because Congress refuses to declare war does not mean that the country cannot be in a state of war... to assert that would be to assert that other countries cannot declare war on the US.
The power to repel invasions is not exclusive to the Congress. That is, it is absurd to insist that the States cannot defend themselves.