Why? You can choose to apply the bonus to one weapon. You don't have to apply it to both. It's not more expensive or ineffective with TWF, it just doesn't double in effect. However, the potential for double the effect exists if you pay double the price.
If want excessive damage with TWF and Arcane Strike, try it with adervish dance.
If want excessive damage with TWF and Arcane Strike, try it with adervish dance.
