All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Offering the reward of the recitation of the Quran to more than one dead person is permissible; it is also permissible to offer the reward to all dead Muslims. Imam Ibn Taymiyyah issued a Fatwa similar to what is mentioned here, but at the same time, he informed that doing what is agreed upon is better than what is disputable.
According to him, giving charity on behalf of a dead person and supplicating for him are preferable and perfect according to the practice of the righteous predecessors.
For more benefit on deeds that benefit a deceased person after his death, please refer to Fataawa 83855 and 84938.
Allah knows best.