A tag is a big investment of resources; if you are a support healer and you do other stuff, you may not have the tag to use; if you are a pure caster, you will almost definitely take this. It bccomes something concrete that separates primary from secondary casters that is based on player decisions rather than external game rules imposed on choice. I like that better. Dual wield also gives you the chance to go all Drizz't. With Might 3, you can wield a pair of longswords 4. That's pretty solid for a blademaster or gladiator type. You are giving up the +1 edge of two handed to have an extra chance to hit for less damage; I feel like that's a good trade. I don't see either as clearly superior. I also don't see this replacing two weapons; some characters will fight with a huge weapon and a smaller side weapon, and others will fight with two equally-balanced weapons. Either choice is viable, so no reason this has to replace anything in the current rules.
Dual Wield. You fight with two melee weapons, receiving one free action each round with your secondary weapon. Neither weapon may be rated batter than your might +1.
Spellcraft. Receive +1 edge whenever you cast a spell from one field of magic (i.e. arcane or light) and with all of your signature spells. You cannot have spellcraft with common spells.
This makes me want to make a pure caster to try it out... maybe my NEXT campaign :)