There actually are already spells that require two schools. Some of them are pretty amazing: Contagnation, Gentle Rain, Earth to mud (or whatever the equivalent name is), forced march. The trouble is that there are so few choices to go around at level up, trying to level multiple schools can seriously hurt your character development.
Perhaps just as importantly, in a huge fit of irony, many of these dual spells are just as effective in the hands of a non-spellcaster focused character. It would make significantly more sense if these spells were restricted to champions with path of the mage, had a level restriction, and did not consume as many level up choices. Really the only reason I can see for the 5 spell schools is to reduce the chances of a champion from getting master level spells too early in the game.
The difference isn't particulary game breaking either: 2 versus 4 level up choices. It just makes it a little more viable dabble in other spell schools. Given that actions in combat are a fixed commodity it won't necessarily make these champions any stronger, but increases the chances of obtaining dual-school spells. To prevent starting with contagnation the spell requirement could be either 2 air or 2 death and 1 of either air or death. Or if that is complicated to program, simply make dual cast spells require a fixed level in schools (1 air 2 death in this case).
Speaking of which, ceresa should really start with air and not fire, as her faction trait doesn't kick until she maxes out the death school otherwise.