That's a very good question. It's hard because Blake insisted that even the most minor detail of his painting was deliberate and meaningful. Everything emanated from on high, but that took the form of a division from one into many, thus the finger pointing in one direction and the other fingers in the other.
I just checked and it's also a perfect 90 degree angle, which is consistent with the theme in that poem (Europe a Prophecy) of Logos bringing order into the universe. He is dividing the divine into parts, and the parts are measured according to a strict mathematical standard.
I don't know if there's something else to be said for this, but it also forms a right triangle with the bottom barrier of the image, and there is an ancient idea (from Plato's Timaeus for instance) that the physical world is made of triangles. The triangle also, perhaps more obviously, represents the holy trinity in Christianity.
I mean this also suggests that perhaps instead of creation it is God the father sending his son and the holy spirit. But it's odd that he would be using his left hand for that since Jesus sits at the right hand of God. And since as we know this isn't strictly speaking God, but a character in Blake's poem.