Balancing the spells doesn't matter at all.

It's either give black mage all spells and give white mage only holy-type spells, or enemies must lose all resistance to elemental damage. Because there will never be a happy medium with the spells split up like this.

If the split continues and the spells stay exactly the same, Black mage will hit a wall against any mobs resistant to lightning. And when white mage sees the wall coming it can laugh and walk around it for the simple fact that "Ok if I can't hurt it, then I'll just heal".

All the while the black mage will have to slam it's face into the wall of resistance.