Welcome to the multiboxing experience!!

I am no expert in the mage field, so don't mark my words. I am pretty sure the spec won't matter much, while leveling, as you will kill stuff so fast anyways. I would say fire/fire however, or maybe fire/arcane for some added utility. Frost is a good spec, but I think it is better suited for longer fights (instances fighing elites). A double pyroblat opening is pretty much devastating.