The subject as witness is the notion that experience is unified in that it is as if always experienced by the one experiencer. One thing that this does not rule out is the idea of free floating phrases or monads of experience each complete with its own subject and no communication between them, perhaps atoms of qualia. Such an idea must have some formal consistency since there are some who take it seriously, but implicit in it is the idea of multiple possible subjects and this idea does not survive the shift to the full subject or witness position. It may be logically consistent but it is also quite meaningless and hence belongs to a 'lower' grade of apprehension. So we begin from the non-dual position that there cannot be two experiences understood in the broadest sense. Kant's synthetic unity of apperception names this peculiar aspect of what can call consciousness. You might contrast this with the analytic non-duality of apperception. Unity is synthetic, it adds something positive, in a sense contingent, but somehow also inescapable. It is this which forces self-recognition through identification, and through this the possibility of there being a world - otherwise why something as baroque as all the folds and branchings, the telescoping penetralia of individual experience.