It cannot always be at conception, otherwise one identical twin would be soulless, or a
human chimera would have two souls. This is not a simple question.
rossum
Definitely not a simple question; here’s how I would put it.
Although the word is a noun, Spirit is not a thing.
It may be thought of as a verb - doing, thinking, feeling and perceiving, but all these require a body to actualized them.
So Spirit is like a capacity.
It is also a unifying principle, in the sense that we exist as individuals, whole and one although composed of diverse parts.
The key word now is existence. We exist as ourselves.
But, being ourselves implies what is other to us that we can perceive, think and feel about, and act upon.
Being is relational, in the sense of a self-other.
We know the known in the knowing. We do something in the doing.
We love, give ourselves to what is other for their good, in the giving. And, it is thereby that we are united with and know them as they are in themselves, as they are known by God.
So all this being and relatedness, our capacity to love, is because we exist in relation to God, who brings us into being, knows and loves us.
Words unfortunately fail but let’s broadly say that creation of a human being involves the “breath of God” passed onto, or perhaps organizing matter as a new being, which is an image of the Triune Godhead, with the capacity to love and the qualities that this requires - knowledge and free will.
We can think of matter not as clumps of stuff, but as data - a type of being that is information. That “information” is its doing what it does to make it what it is. Chemicals do stuff that is basic. In becoming one in the form of a person, matter can be said to inform the spirit. It is also the means by which the spirit expresses itself in time and space. The being and doing of chemicals becomes one under the organizing principle that is the person.
Bearing this in mind, an analogy may be made to a server in a computer network. The matter that constitutes the zygote provides information to the spirit and in the case of twins, contributes to two different consciousnesses.
Their individual God-given existence in one embryo would be the cause of its splitting.
Again, the spirit and the body are one. While there would be a closeness in having shared the same body at its earliest stages, it is still love that would join the twins. A compassion stemming from the knowledge that the other is just like themselves. But that is the case for all of us in a way, as children of God, existing in His image, feeling, thinking, perceiving and doing.
That said, the reality of who we are, as beings grounded in eternity, is actualized through our decisions, what we do with what we are given.
As long as this post is, it’s the short of it.