(2) Such increase shall be payable in accordance with the following provisions:
(a)where a retirement pension or a survivor’s benefit is payable only under the legislation of one Party, the increase shall be payable under the legislation of that Party;
(b)where a retirement pension or a survivor’s benefit is payable under the legislation of both Parties and an increase would otherwise be payable under the legislation of both Parties in respect of the same child, the increase shall be payable under the legislation of the Party in whose territory the pensioner, widow or widower is ordinarily resident at the date of entitlement to the increase.