The Primarch's gene-seed would not be compatible with the genes of a girl, and the two different genes would destroy each other.Young girls would die right out from the procedures. You need physically fit candidates, and one's that have strong bodies at a younger age. Most male neophytes die during the organ plantation.Woman would not be ideal candidates to be Space Marines for several reasons. I believe in one of the books in the Horus Trilogy it briefly discusses when the Emperor found the Primarchs and knew them both to be twins. So, the Emperor was aware of the twins but kept it secret for unknown reasons.
It may have not been planned for the embryo to split, but that's what it did and it had to have happened before Chaos threw the Primarch's across the galaxy. The Emperor did know of Alpharius Omegon, remember that he created both of them.
They made one before, but that chapter was corrupted by Magnus himself.
The closest that anyone got to creating new gene-seed was actually the Space Wolves, but Magnus attacked Fenris and destroyed the research lab there - forever keeping the Space Wolves from creating any successor chapters. The Imperium has actually lost the technology required to create new gene-seed, especially on the level of a Primarch.
