IANAL, but the wikipedia page on Interstate Compact states that "Not all compacts between states require explicit Congressional approval - the Supreme Court ruled in Virginia v. Tennessee that only those agreements which would increase the power of states at the expense of the federal government required it."
I'm not sure this would cause the federal government to lose any power, especially if the ACA was repealed.
Also, if the bill allowing for interstate healthcare exchanges were passed it would seem to imply that states are allowed to cooperate on healthcare?
It would be great if it goes this way. I would guess that the Feds would sue to stop it in spite and to protect insurance companies and then it would go to the Supreme Court.