The duly constituted administrative authorities of any 2 or more party states may enter into supplementary agreements for the provision of any service or facility or for the maintenance of any institution on a joint or cooperative basis whenever the states concerned shall find that such agreements will improve services, facilities or institutional care and treatment in the fields of mental illness or mental deficiency. No such supplementary agreement shall be construed so as to relieve any party state of any obligation which it otherwise would have under other provisions of this compact.   [PL 1983, c. 459, §7 (NEW).]