I assume you guys have the Backstab and Assasin's Blade perks? Without them you just get a critical hit, which isn't that impressive with a dagger.
The animations play when the victim dies from the damage done, not vice versa. I get either the troat-slitting or kidney-stabbing animation at about half of the sneak attacks done with one blade (i.e. one button pressed, even when dual wielding)
I never get an animation when doing a double-sweep. Seems like the x30 damage of a double-dagger backstab is so devastating that it sends most victim's corpses flying away before my char can grab them.
What's really strange for me..
in the heat of battle vs a mean guy in a cave.. i was doing the whole panic'd run away because a single blow from him was killing me mostly due to being such a lowbie at the time
i managed to whirlwind myself around a pillar and next thing i new i was at his backside as he was spinning around to hit me, i managed to get a jab in with my dual daggers... only the moment i hit the key, it cut to a quick action scene in which i stuck both into the kidney zone and proceeded to remove his head at the neck...
I'm not entirely sure.. but i haven't been able to do that since.. i had ZERO sneak skills, and really no progress into the game, it was pretty damn epic cutscene.... but seemed like it wasn't appropriate for what i did.