The said Act shall be construed as extending to authorise any person seised or entitled only for life or lives of or to any manor or lands of freehold tenure to make such grant, conveyance, or enfranchisement as is mentioned in the said Act in cases where the person next entitled to the same for a beneficial interest in remainder in fee simple or fee tail is unborn or unascertained: Provided that no such grant, conveyance, or enfranchisement made by any such person seised only for a life or lives shall be valid unless the person seised or intitled for a benefical interest for life or lives, or for an estate in fee simple or fee tail (as the case may be) in remainder immediately expectant on the estate of such unborn or unascertained person of or to such manor or lands (if any, and if legally competent) shall be a party to and shall join in the same; and if there be no such person, or if such person be not legally competent, unless the trustees or trustee (if any) of such manor or lands during the suspense or contingency of the then immediate or expectant estate in fee simple or fee tail in such manor or lands shall in like manner concur.