Which of them has the right to conceive first: the first wife, from whom I have a son and a daughter, or the second wife, who is a martyr's widow and has a son from him?

There is no religious prohibition against procreation; it is a right for both spouses, and neither should be prevented from it unless there is a legitimate reason. This is because the wife has a right to have children, and she is harmed if her husband practices azl (coitus interruptus) without her permission.