He could just take some woman's fertilized egg, replace it's nucleous by the nucleous of a cell from Veronica's body (thus, transferring her DNA). This cell will have Veronica's genetic code and so, it should generate a clone ( an identical twin ) of her. Of course, then he has to insert the egg in the uterus of a woman, so the egg has the conditions to grow and become a human being.
Now, this theory fails because the twins are blonde and one of them is a boy. Hair colour and sex are both genetically defined so, if they have the same genetic code as Veronica, they should have black hair and be two girls. Also, they wouldn't be identical twins (although they have the same genetic code, so their appearence might be identical).
The alternative is that Alexander used an oocyte (not fertilized), replace it's nucleous by a nucleous from Veronica's cells, and then fertilized it with his own sperm cells.
This is more coerent, it explains the differences between the twins, the fact they are blonde and in this case they can be identical twins of each other.
The only thing against this is that i think Alexander wrote that he cloned Veronica. In this case, he didn't clone her, but "reproduced with her".