If am agreement is made between the Federation and one or more Provinces, or between two or more Provinces, for the maintenance or creation of a service common to the Federation and one or more Provinces, or common to two or more Provinces or for the maintenance or creation of a post the functions whereof are not restricted to the affairs of the Federation or one Province, the agreement may make provision that the Governor-General or any Governor, or any Public Service Commission, shall do in relation to that service or post anything which would under the provisions of this chapter be done by the Governor or the Provincial Public Service Commission if the service or post was a service or post in connection with the affairs of one Province only.