Until such a time as the question of relative caster level is answered, I'll just operate under the assumption that the source of the sphere is my only access to that sphere and thereby the only class with which I can gain associated talents.
Makes sense that way, at any rate.
Another option for consideration is home brewing a feat or two as an answer. One that is tailored to making two classes mesh a little more organically while still requiring commitment of resources to do so.