All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
We think that it is our duty to trust in Allah and to continue in procreation since everything is in Allah's Hand. If your two sons have blood diseases, this does not mean that a newborn will have the same disease. Also, are these diseases transmitted to your children? Then, if such a disease afflicts your son, perhaps there will be some treatment even it needs to change your son's blood. That is better than stopping procreation, which constitutes a form of worship and a means towards prosperity of the Muslim Ummah, especially if such children are pious and might be beneficial to their parents and to the Ummah.
Allah knows best.