If u wana do caster its best u stay pure, ive seen a lot caster who multiclass arcane-arcane.
Especially
wizard splash sorcerer
or
cleric splash FVS
no matter how u weigh it, its not gona be better than staying pure
exceptional would be like wiz/rog, divine/monk, the best i think can think of but dont do other kind of splashes its gona ruin ur end game capability, plus it doesnt do any benefits