If someone memorized half of the Qur'an (from Surah Al-Fatiha to Surah Al-Kahf) in Hafs 'an 'Assim, and then memorized the other half (from Surah Maryam to Surah An-Nas) in Warsh 'an Naafi'. Does he still count as a Hafiz or does he have to repeat memorizing the first half in Warsh?
I don't think there's a clear answer. But I'd say no it is not possible unless you become an illegal hafidh! (Meaning yes you memorized the qur'an, but from a book not from a teacher or you've lied to your teachers to become such).
You may read in the long hadith qudsi about the first of men (whose case) will be decided on the Day of Judgment that of the person who became a scholar just in order to be called such:
... Then will be brought forward a man who acquired knowledge and imparted it (to others) and recited the Qur'an. He will be brought And Allah will make him recount His blessings and he will recount them (and admit having enjoyed them in his lifetime). Then will Allah ask: What did you do (to requite these blessings)? He will say: I acquired knowledge and disseminated it and recited the Qur'an seeking Thy pleasure. Allah will say: You have told a lie. You acquired knowledge so that you might be called" a scholar," and you recited the Qur'an so that it might be said:" He is a Qari" and such has been said. Then orders will be passed against him and he shall be dragged with his face downward and cast into the Fire. ... (Sahih Muslim)
In the Arabic language hafidh الحافظ refers to a person who is a keeper, a guardian, a protector or memorizer etc. (see also Who exactly can be called a hafidh?)
In the context of qur'an hafidh refers to a person who has memorized the qur'an fully.
So in theory it is possible to become a hafidh by mixing several riwayat. But this won't be an acceptable way of becoming a hafidh:
But this requires finding a teacher who himself is a hafidh and allows this. As to be a hafidh you can't just memorize the qur'an from a book as the qur'an is only orally transmitted you need to find a teacher who is a hafidh and has knowledge on different riwayyaat.
In practice this is considered as none acceptable. You won't find a teacher who will allow you to do this. Because the process of memorizing is hard and you will cause yourself many problems to progress while memorizing so the usual way to become a hafidh of different riwayyat is to excel one initial riwayya and this means you must become a hafidh of this riwayyah first.