I’m not sure how that works...but an interesting question if they can do this.
Found this....
Except as limited by the Constitution, the several states may enter into any compact, agreement or other arrangement as they choose, provided, the states cannot limit or surrender by such compact or agreement, the sovereign rights of the people.
Why would it be a treaty? We have private trade deals all the time. People don’t always need the permission of government to do something.