So, today in a game I was in, I ended up fighting against some Giant creatures, and they ended up giving me a hug (grapple). To free me, my friend put it to sleep, I climbed out and committed a Coup de Grace. I talked to my GM and they said they did not know about using both weapons on it, and well here I am.
By my logic, since they use the same attack bonus, the pretty much happen at the same time. In honest, it is like adding a rogue's backstab in my opinion, since they can't really fight against the sting of both blades being helpless.