This is a fairly powerful question on a very intense work sample. I am not sure any one person can claim to have "the answer" for any question in a work like Eliot's, so be warned on this front. In my mind, Eliot's use of religious figures from different venues as Christianity and Buddhism is a reflection of the Modernist style in the work. Eliot is seeking to broaden understanding in making the argument that the traditional methods of establishing meaning have failed. Christianity is a part of this, and its failure is something that drives Eliot in the work. The rigid function and dogmatic laws of Christianity cannot offset the decline and crisis that exists in the modern setting for thinkers like Eliot. The inclusion of Buddha might be a call to demand new ways of thought in such a moral and psychological abyss that looms over the lives of the people in the poem. Additionally, Eliot might be suggesting that Augustinian edicts cannot prevent the sense of disillusion in the modern setting and to invoke someone from a diametrically opposite religious point of view as Buddhism might be a way to highlight this. If one didn't like this read, perhaps another, and more drastic, one could suggest that both thinkers seek to overcome the world of the mortal, but cannot. The section in which Eliot uses both figures is one where individuals are pitted between the crushing nihilism of the modern setting set against the desire to be free from it. The act of sex, which is present in the section, is in its own right an act that is both of this world, and one that strives to be free of it. The physical ecstasy of sex, or its perception, drives one into its clutches, but each of the characters in the poem are cursed in that the more mundane and banal aspect of sexual activity reflect a fundamental sense of waste and lack of transcendence. To this end, both religious thinkers might be exactly representing this, and to bring them both together could be a way for Eliot to suggest that no matter the thought or the philosophical leaning, crisis and disillusion follow all individuals.