[jandrewrogers]

These kinds of pacts between States to coordinate on specific issues that are under of the umbrella of their sovereignty are quite common and perfectly legal. The whole point of sovereignty is that they have the freedom to deal with the issue as they wish as long as it doesn't abrogate one of the narrow powers delegated to the Federal government, such as this case. There are even pacts between States to coordinate action directly against Federal policy e.g. the Sagebrush Rebellion.

In other cases where the Federal government does have jurisdiction, such as interstate treaties regarding shared resource rights, the Federal government can delegate the issue to a group of affected States to sort out amongst themselves e.g. the Colorado River Compact of 1922.