The Double Kill.
I'm not sure if you guys figured out how to do it yet, I'll have a video showing it soon (this is not the double hidden blade kill)
The short video showing it:
http://www.youtube.com/watch?v=hH_xxAvdEoo
I'm not sure if I know the right way of doing it, so if you know the 100% sure method of doing it, feel free to post a comment.
P.S. The double kill is at about 10 seconds.
It's pretty easy. When in the midst of a killstreak, hold the attack button instead of just tapping it. This will yield a double kill, provided you have ammo.
I found out that you can get into a killstreak, then when there are civilians close to the fight and 1 guard is left, perform the double kill on the lone guard and you'll shoot the citizen. It's just fun to see it actually happen to some old, fat woman. 
Heh, I got desynched by doing do that before. lol
I think I show it in a few of my VR kill streak vids
HA! Thanks for this, I didn't know about holding down the button for a double 