It can't. For the Coordinator to set the PAN ID on a remote module, the PAN ID must be 0. Then and only then can the coordinator select both the 16 bit and 64 bit PAN ID's. Otherwise if you set the 16 bit PAN ID, then the Coordinator must use the same 16 bit PAN ID. If then both 16 bit PAN ID's are the same, the coordinator will set the 64 bit PAN ID during the association process.