I jumped two guy thinking I'd knife them both, but by the time the animation from the first kill had finished the second guy had moved away and shot me, so it seems like there needs to be a rethink in strategy for these situations. shoot one guy knife the other perhaps?
I've also read conflicting comments about how the knife works, but this seems to be the most comprehensive one I've found
I'm still not sure it's correct though. I had my knife key bound to a mouse button and used that for both my kills. In both cases I have the dog tags from the players, and from memory both were one hit kills. One was a sniper I snuck up on, the other I sneaking up on but he turned around and it ended up being a face-on kill (after which I got shot)
One thing I'll have to look for
The knife has a special little feature for new users, if there is no target, or target is not in range, then the knife points vertically down when equipped. However when the target becomes in range the knife often moves to a horizontal position to indicate that the target is within range, this makes it much easier for newcomers to know when to knife (the range for selected knife attack is greater than quick knife, so do not press quick knife and remember that in a knife fight).
I think more practice is required.