A second parallel is that reality is primary, in metaphysical terms, before unreality: unreality can only be created from reality, but not the other way round. Every form of unreality relies on a background of reality which is much larger than itself. (For instance, take a fictional story, or a lie: we take in some description of the world in those, but most of that world is not explicitly described; so whenever there remains a gap in the description, we either fill it from what is implicit in it, or else we fill it in from what we assume to be the case in the real world.)
Sartre claims something similar when he says that "[t]he use which we make of nothingness in its familiar form always supposes a prelimiary specification of being." And he continues with some examples:
We say, pointing to a particular collection of objects, "Touch nothing," which means, very precisely, nothing of that collection. Similarly, if we question someone on well-determined events in his private or public life, he may reply, "I know nothing." And this nothing includes the totality of the facts on which we questioned him. Even Socrates with his famous statement, "I know that I know nothing," designates by this nothing the totality of being considered as Truth.Even the nothingness of what was there before a world existed would be based on the world which is now, and from within we can ask such a question. Such a nothingness (the 'nothing' we mean when we answer the question: "What was there before our world?" with "Nothing.") emerged only on top of our reality. If we did analyze it and strip it from "its characteristic of being empty of this world and of every whole taking the form of a world" as well as from its "characteristic of before, which presupposes an after", then we would end up with "a total indetermination which it would be impossible to conceive, even and especially as a nothingness."
"This means", Sartre concludes, "that being is prior to nothingness and establishes the ground for it. [...] nothingness can only have a borrowed existence [...], and the total disappearance of being would not be the advent of the reign of non-being, but on the contrary the concomitant disappearance of nothingness." There can't be any nothingness without being (or before, or after it), just as there couldn't be any unreality without reality.
(As a side-note: the process I have labeled sedimentation of unreality into reality also relies on this grounding of unreality in reality. Sedimentation happens when on the basis of some instance of unreality action is taken, in reality. Real events happen in response to unreality just as well as they are caused by something within reality. But all this presupposes an underlying reality as basis on which that unreality was formed. There is a hint to a parallel to this also in Sartre when he remarks that "it is from being that nothingness concretely derives its efficacy.")
 Being and Nothingness, 48–49.
 Ibd., 48–49.