Shattered Kingdoms

Does it happen if your opponent is on kill and not stun? Doubtful in my mind.

Yes. At least it happened a few times with Rokki and backstabbing golems. Don't know about now.

Since the double backstab is a second attack on it's own, counterstrike will never recognize it. Since counterstrike, and double backstab always happen instantly, the second backstab will always advoid the countstrike because counterstrike only looks at the first damaging attack (though you can still dodge/block/parry a double backstab) but I believe it will always go through reguardless if you are stuned or dead because it's happening at the exact same moment as the counterstrike.

So, you have 3 actions happening instantly at the exact same time, and unless there was some way for the game to check to see if counterstrike will stun or kill you before you can double backstab, the problem won't go away. You could always merge double backstab and normal backstab into one skill, kinda like how tame for scouts change when you level up, and let a rogue double backstab when they reach an appropriate level. Then let counterstrike return the damage for both backstabs, though chances are it'll insta kill rogues.

I think the counter should instantly stop the second stab. You go to backstab someone and right as the knife is about to enter - they turn around, parry the attack, and jab you in the face. The ninja of the barb should be enough to initiate melee. Not to mention that at that point he would be turned around.

Nah, you counter it with a devastating mule kick to the groin... that kills!

What I want to know is, if the FIRST backstab misses, will the SECOND one get countered?

I think it still counters the damage the first backstab would have done even if it misses, but the second one still gets through.

Countstrike isn't always a win skill anyways, I've seen rogues shield block or dodge my counterstrike on a few occasions.

The only way that this wouldn't be a bug is if dual backstab required wearing two daggers. As it is, if the rogue is stunned/killed on the first backstab, they wouldn't be able to plunge a second deadly attack. Maybe something at 1/4 the strength due to gravity taking effect on the falling rogue. And even then, the accuracy would be horrible.

Maybe with his last final breath, the rogue throws the dagger with devastating accuracy. The bug then would be that the dagger remains with the rogue, when it should be transferred to the inventory of the barb.

Or maybe the rogue went back in time to detonate a nuc-- err to backstab him while he was backstabbing him in the past thus avoiding the counterstrike (the present he, not the past he).