Nonduality means "not two," but "not two" also means "not one." That is why we say "nondual" instead of "one." Because if there is one, there are two. If you want to avoid two, you have to avoid one also.
In the Satipatthana Sutta, the basic manual on meditation from the time of the Buddha, it is recorded, "The practitioner will have to contemplate body in the body, feelings in the feelings, mind in the mind, objects of mind in the objects of mind." The words are clear. The repetition, "body in the body," is not just to underline the importance of it. Contemplating body in the body means that you do not stand outside of something to contemplate it. You must be one with it, with no distinction between the contemplator and the contemplated. Contemplating body in the body means that you should not look on your body as the object of your contemplation. You have to be one with it. The message is clear. "Nonduality" is the key word for Buddhist meditation.
In short, we have to be one with the contemplated (to attain insight), but we should not stand outside of the contemplated (to observe by eyes).
* Satipatthana (The four foundations of mindfulness)
1. Body (Kāyā)
2. Feelings (Vedanā)
3. Mind (Cittā): (51 mental formations)
4. Objects of Mind (Dhammā): (‘nature’ for the scientist)
* Mara and the Buddha (The nature of non-duality)
Thích Nhất Hạnh