Dual Blade Guide (Chaos, Big Bang, and Pre-Big Bang)

In first job, it's best to not use double strike (or whatever), because it's essentially the same as when you use your normal attack with a blade+dagger. Attacking is moderately slow until you get a blade, but the 20 SP is better spent on Nimble Body + Dark Sight rather than on attacks you'll never use again.
EDIT: It also leaves more SP for Fatal Blow, which isn't so useless.