I have done a Double Assassinate from a parachute about 10 times now and it has not given me any achievement. does any one know why or am i doing somthing wrong?
You need the Da Vinci Disappearance DLC to get these achievements.
If you check your gamer score on the Xbox, you'll see these achievements after the Multiplayer achievements (IF you've not gotten them) or here on Xbox.com. That means they're DLC.
Imagine if there was an achievement for triple assassination xD
[quote user="KiiD x NiiNjA"]
I don't want to know where that third blade would come from...
lol you and me both