If an opponent has a number of 2 weapon talents, and makes 2 attacks (a 1/2 action) against one opponent, but the first attack decimates that opponent, what happens to the 2nd attack? Is it wasted? Can he/she shift it to another target. the whole dual attack talent lets you make one roll for both attacks.

I'm almost certain any action that allows for more than one attack is a full round action, not a half action. In Black Crusade this isn't the case (but different system), also, 2 weapon wielding still ends up as a full round in that too, right?

Two weapon wielder makes use of the Multiple Attacks action, which is a full round action.

Reading further into the post though, it appears you are confusing Dual Blow with Multiple Attacks. Dual Blow (not sure if I got the wording right, don't have my book) is intended as a way to deal more damage (which is superior to multiple, weaker attacks in this system given the DR nature of TB+AP), at the cost of making it a single attack (easier to dodge). To answer your particular question in regards to Dual Blow, then no, you most certainly lose the second attack, as its part of what makes the single attack so powerful. But it isn't the "standard" means of attacking with Two Weapons Wielder.

On "normal" Two Weapon Wielding with the Multiple Attacks action, I at least rule that a new target in melee range can be engaged with the remaining attacks, however I do not allow movement (unless they're using Assassin's Step, which should allow it to be done mid Multiple Attacks/Lightning Attacks).