I'm still trying to find out what determines a slit throat animation, but I've gotten it to happen more often with a single, regular dagger attack from behind. Power attacks seem not to prompt this finisher move, though 1h Power attacks may still work. I've never gotten a dual 1h power attack to result in a slit throat finisher.
The upside to the slit throat animation is that you're discovered or seen a LOT less. When I've killed enemies in other ways that didn't result in a slit throat finisher, I found I was seen much more often.
It also requires the target npc to be standing and you to be on even level. IE you can't perform one if you're both on steps or if they're sitting down.
Having two daggers ready does help though if you miss or don't do enough damage to kill them outright. Then you can just spam dual 1h power attacks 'till they're dead.
For this question, Yes and Yes.
VBane,
Thanks man!
I found the same thing, if I hold down the R1 button (to get right hand power attack with dagger) I would get the slit throat. If I held down both R1 and L1 (PS3 dual power attack) I would never get the slit throat and he'd just do that crazy dagger spin move, cool, but would never get the slits with this, so here's what I'm guessing PLEASE CORRECT ME IF I"M WRONG!

1. Approach enemy from directly behind.
2. Hold down R1 (right hand 1h dagger power atttack) Hopefully get a slit throat. IF DOES NOT DIE: 3.
3. Now HOLD DOWN BOTH R1 and L1 to do the DUAL POWER ATTACK spinny crazienes attack to finish off?
So is that the best, and most efficient way to take out a npc at a super sneaky rogue with two daggers?