Well, it is:
Article 1, Section 10:
No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
newfreep you are right. These arrangements amount to compacts between and among the states, which is constitutionally prohibited.
An analogy is price fixing. Suppose I make a statement that I will keep my price at competitive levels until my rivals all raise their prices to the monopoly level. But as soon as these rivals all raise their prices to monopoly levels I will follow suit.
I am not entering into any formal contract or compact with my competitors to restrain competition, but 100 out of 100 judges would call this naked price fixing and send me off to jail.
All existing compacts between States involving boundary issues, water rights management, power, etc. have been executed with the permission of the Congress. This compact would be subject to the same caveat, it would require the approval of Congress. Since this would destroy the political power of all of the small states, it’s very unlikely to receive Congressional approval even if the Democrats controlled both Houses. This stuff doesn’t count, a freebee for Democrat controlled states. Interestingly, the Democrat Governor of Nevada vetoed this compact. Some Democrats get it.