I've tested it multiple times in the training area and the survival mode. Can't get the second combo hit to land on an opponent who's vulnerable. It doesn't help that the first hit being the one to break an enemy's guard will interrupt your combo to knock them back, but even so, following up by whiffing the first swing means the second attack can pass all the way through the opponent's body without registering as a hit. With the right positioning, a parry into a combo has the same result. Opponent is knocked back by the parry, out of reach for the first swing of the combo, but the second attack passes through them without killing, and it's always the third hit which kills.
And when I say "passing through" it can be anything from the tip slicing through the front of the body where it should be catching the front few pixels of the torso, to the blade being visible sticking out behind the opponent, and nothing works.
Might be a hit detection problem in the current build? It registers for enemies blocking it, though.