I've been pondering this question a lot, too, and while I can't say I have the "right" answer (I don't know if there even is one, to be honest,) I have come to a few conclusions that seem to make sense. Ultimately, everything co-exists. So duality cannot exist without unity and vice versa. For example, If we accept the current scientific theory of how the Universe was created, aka the Big Bang, then "everything" (these are not necessarily absolute terms) literally came out of "nothing". From a single concentrated point in space, all that constitutes the cosmos flew out into being. Can you see how coexistence applies here with the seemingly separate (but not realistically so) ideas of "one" and "all"? Now to more directly address your question, the mind-stream is both individualized & localized while also being non-individualized & non-localized. It fits in with the five aggregates: material form, feelings, perceptions, mental formations, and consciousness. We all share these things, but have different "kinds" of each. So, we all have material bodies which are composed of the same elements, and yet they look different. We all have feelings, perceptions, and thoughts, and yet they are unique to each of us for many different reasons, the main one being we are different people with different life experiences (on a conventional level). Each of these five aggregates is inherent to this realm of samsara, and as such are impermanent. Because our consciousness is constantly in flux, it cannot be said that there is one, pervasive consciousness amongst everything because that would suggest no change and no difference in conscious states. Because there is no self, when you die "your" consciousness is not reborn, just as "your" body, feelings, perceptions, and thoughts are not reborn (at least holistically, but that's worthy of another posting) but consciousness (and each of the other aggregates) in and of itself is. I really hope this makes sense, hehe.
Peace comes from within. Do not seek it without. - The Buddha