All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Waqf (Endowment) is among the acts of obedience by which a Muslim draws closer to Allah the Almighty. A Muslim offers this kind of donation in the form of an after-death continuous charity. This is attested to in the following narration: Narrated Ibn `Umar: When `Umar got a piece of land in Khaibar, he came to the Prophet (PBUH) saying, "I have got a piece of land, better than which I have never got. So what do you advise me regarding it?" The Prophet (PBUH) said, "If you wish you can keep it as an endowment to be used for charitable purposes." So, `Umar gave the land in charity (i.e. as an endowment on the condition that the land would neither be sold nor given as a present, nor bequeathed, (and its yield) would be used for the poor, the kinsmen, the emancipation of slaves, Jihad, and for guests and travelers; and its administrator could eat in a reasonable just manner, and he also could feed his friends without intending to be wealthy by its means." {Bukhari & Muslim}.
It is well known that the benefits of the Waqf are divided amongst beneficiaries while observing the conditions of the donor. An-Nawawi (May Allah be pleased with him) said, "The condition of the donor regarding shares, characteristics of recipients, and time of reception should be taken into consideration. If a person endowed a property in favor of his children and stipulated equality between males and females or favored one gender over the other, then his condition should be fulfilled….... If he said: "I endow that property in favor of my poor sons or widowed daughters, then those sons who become well off or the those daughters who remarry aren`t entitled to the endowment. But, if the sons become poor again and the daughters become widowed or divorced, then they become re-entitled to the yield of the Waqf." {Rawdat-at-Talibeen, Vol.5:P.335-338}.
In conclusion, if one of the Waqf beneficiaries meets the conditions that entitle him/her to receive two shares from the revenues-a share from his/her father and another from his/her mother-and the donor hasn`t clearly stipulated that such thing is prohibited or that all beneficiaries should get equal shares, then this is permissible. And Allah the Almighty knows best.