Christian theologians: Do twins have one spirit or two? Did "vanishing twins" have two spirits or one?

I should preface this by saying that I am an atheist, although I am not a materialist. So while I discount the existence of a god or gods, I am open to the possibility of, for example, spirits. (I use Gnostic terminology here and in the question; we have a spirit, we ARE a soul. In Gnostic theology, the soul is the combination of body and spirit—that is, soma and pneuma.)

I am curious, therefore, about the theological implications of the christian doctrine that human life begins at conception. There are a number of cases I’d like to put to the various flavours of christians here to get your take on the theology of it, and what biblical supports you can provide for your view.

(1) Identical twins start out as one cell at conception. At some point, they develop into two zygotes from a single cell ancestor. If we accept that “human life” (ie/ the inception of a spirit) occurs at the moment of conception, does this mean that identical twins have only a single spirit between them? If not, what determines which is the “original” spirit, and where did the other spirit originate?

(2) In the case of a “vanished twin” (known medically as foetal resorption), one twin absorbs the other in the womb. If in answer to the first question we say that each twin has its own spirit, what happens to the spirit of the absorbed foetus? And does the resulting baby have one spirit or two?

(3) Chimeras are the result of a similar phenomenon to foetal resorption, except that rather than a twin which is re-absorbed, it is two genetically-different entities which cooperate to form a single entity. For example, two fertilized eggs may collaborate with each other to form a single entity, half of whose cells carry one genetic legacy and half the other. This is extremely rare, but it is possible and has occured. Again, if spirits originate at the moment of conception—and here we have undeniable evidence that there were two discrete individuals—which of the two spirits does the single entity have? Or does the chimera have two spirits in one body?

Please provide scriptural support for your view.