In Brotherhood Ubisoft invented this nice combat that you can kill the enemies next to you if you once killed an enemy, as long as nobody hits you.
During this kill streaks, you can also perform a hidden gun kill or knife kill by holding left mouse button for about a second, and see: Ezio kills the enemy next to him and shoots a knife (with short blades) or with his gun (usual blades) to another enemy.
The good thing about this was not only that you can kill two enemies at the same time; as long as you perform this shooting combat streaks you can't even get hit by your enemies.
Does anybody got performed this in Revelations? I can't perform it by holding left mouse button, even if I have selected hidden gun or knifes as my secondary weapon.
It seems Ubisoft has removed this a bit overpowered invention; bad idea because I really liked it and once you've discovered it, it really helps you during combat. In the Training Program of Brotherhood I got a kill streak with about 35 kills on this way.
I've had trouble with this as well -- really loved that, especially the knife throwing animations in Brotherhood. Brotherhood was by far the best game of the 4 released so far. I miss subtle combat system differences between Brotherhood / Revelations.
If anybody is still interested: when I played the single player recently I just noticed that it works indeed now – I guess this feature has been delivered with some of the patches, because I'm sure it didn't worked at the beginning with 1.00.
It works same as in Brotherhood: you simply have to hold down the left mouse button for a second and press WASD (don't know what it is on consoles) while in a kill streak to perform a hidden gun double kill or a knife double kill (depending on if you're using sword or knife). You don't even have hidden gun or throwing knifes to be selected as secondary weapons.
It seems not to work with janissaries, but with any kind of "usual" guards.
Hope I could help anybody who's searching for this feature!