Casting Ability Modifier
If a caster takes levels in multiple casting classes, they must select which of those classes’ casting ability modifiers they will use as their casting ability modifier; only one may be applied. Multiple Traditions is an exception to this rule, and may be found in Player Options.
So you don't add two casting ability modifiers, because you don't have two casting ability modifiers.
If a player or GM wishes a player to follow two traditions (similar to creating a wizard/cleric in traditional Pathfinder), they may do so. When gaining a level in a casting class beyond their 1st, the caster may apply that casting level to a new tradition. They gain an entirely separate set of spheres, talents, caster level, spell points, drawbacks, and boons, which are determined similarly to but completely separate from the first set. When creating a magical effect, they must choose which of these traditions (and its associated spheres, talents, spell points, etc.) they will use; they cannot use aspects of one tradition to empower the other.
Whenever the caster gains a level in a casting class, they must decide which of their traditions that casting class level will be applied to for gaining spell points, casting level, and talents.
So with Multiple Traditions, you can add two casting ability modifiers (or even the same ability modifier twice). But it comes at the cost of not stacking caster levels, not being able to combine one set of talents with the other set, and a few other finicky restrictions.