Madhyamaka is advayavada (from Nonduality, A Study in Comparative Philosophy, by Prof. David R. Loy, 1988, Amherst 1998)
Advaita Vedanta clearly asserts nonduality in our third sense [the nondifference of subject and object], to the extent of making it the central tenet. The case of Buddhism is more complicated. Ontologically, Pali Buddhism, which bases itself on what are understood to be the original teachings of the Buddha, seems pluralistic. Reality is understood to consist of a multitude of discrete particulars (dharmas). The self is analyzed away into five ‘heaps’ (skandhas) which the Abhidharma (the ‘higher dharma’, a philosophical abstract of the Buddha’s teachings) classifies and systematizes. So early Buddhism, while critical of dualistic thinking, is not nondual in the second, monistic [the nonplurality of the world], sense. Regarding the nondifference of subject and object, the issue is less clear. While the second sense of nonduality [the nonplurality of the world] logically implies some version of the third [the nondifference of subject and object], it is not true that a denial of the second sense implies a denial of the third. The world might be a composite of discrete experiences which are nondual in the third sense.
I am not acquainted with any passage in the Pali Canon that clearly asserts the nonduality of subject and object, as one finds in so many Mahayana texts. But I have also found no denial of such nonduality. One may view the no-self (anatman) doctrine of early Buddhism as another way of making the same point; instead of asserting that subject and object are one, the Buddha simply denies that there is a subject. These two formulations may well amount to the same thing, although the latter may be criticized as ontologically lopsided: since subject and object are interdependent, the subject cannot be eliminated without transforming the nature of the object (and vice-versa, as Advaita Vedanta was aware)..
Mahayana Buddhism abounds in assertions of subject-object nonduality, despite the fact that the most important Mahayana philosophy, Madhyamaka, cannot be said to assert nonduality at all, since it makes few (if any) positive claims but confines itself to refuting all philosophical positions. Madhyamaka is advayavada (the theory of not-two, here meaning neither of two alternative views, our first sense of nonduality [the negation of dualistic thinking] ), rather than advaitavada (the theory of nondifference between subject and object, our third sense). Prajña is understood to be nondual knowledge, but this again is advaya, knowledge devoid of views. Nagarjuna neither asserts nor denies the experience of nonduality in the third sense, despite the fact that Madhyamika dialectic criticizes the self-existence of both subject and object, since relative to each other they must both be unreal: “Nagarjuna holds that dependent origination is nothing else but the coming to rest of the manifold of named things (prapañcopashama). When the everyday mind and its contents are no longer active, the subject and object of everyday transactions having faded out because the turmoil of origination, decay, and death has been left behind completely, that is final beatitude.” (Chandrakirti, Prasannapada)