When a disposition under a power is directed to be made by, between or among several persons, without specifying the sum or share to be allotted to each, all the persons designated are entitled to an equal proportion; but when the terms of the power import that the estate or fund is to be distributed between the persons so designated, in such manner or proportion as the trustee of the power may think proper, the distribution or apportionment made by such trustee cannot be impeached on the ground that it is unsubstantial, illusory, or nominal.