In the novel, there is no mention about life stealing or something like that.
Here is all mentions about the Dual Blade skill
During the fight with the 74th floor Boss - The Gleam Eyes
With the sword in my right hand, I blocked the demon’s blade as it
came down from a flame-like trajectory. Then, I reached behind my back
with my left hand and grabbed the handle of the new sword. I drew it
and struck in one smooth movement.
This time, I crossed my two swords and blocked it completely. As its
stance became unbalanced, I decided to break free from my defending
streak and launched a combo attack.
My right sword slashed horizontally towards the demon’s midsection.
My left sword followed immediately to cut vertically into its body.
Right, left, then right again. I swung my swords as if the nerves in
my brain were going into overdrive.
This was the extra skill that I had been hiding, «Dual Blades», and
the technique I was using was its high-class sword skill «Starburst
Stream», a sixteen-hit combo attack.
Faster, faster. The rhythm of my swings already exceeded twice the
normal speed, but it still felt slow to my heightened senses. I
continued my attack at a pace that seemed to overtake even the
Most of the ten-plus extra skills that had been found up until now,
«Katana» included, had at least ten people who used each of them. The
only exceptions were my «Dual Blades» and one other man’s extra skill.
These two were most likely limited to only one person, so they should
be called «Unique Skill».
Since then, I only trained it when there was nobody around. Even after
I nearly mastered it, I rarely used it against monsters unless it was
Fight agaist Heathcliff (in the stadium)
I twisted my body around right before reaching Heathcliff and swung
the sword in my right hand upwards to the left. It was blocked by the
cross-shaped shield and sent out a burst of sparks. But my attack was
part of a two hit combo. Point one second after the first strike, my
left sword slid in behind the shield. It was a Dual Blades dash-type
skill «Double Circular».
I just managed to guard by crossing my two swords. The powerful impact
rattled my entire body and sent me flying back by several meters.
After defeating the Skull Reaper, the last fight
Out of the ten unique skills, «Dual Blades» is given to the player
with the fastest reaction speed(...)
I changed my attack pattern and activated «The Eclipse», the highest
level Dual Blades skill. Like the tips of an enveloping corona, my
swords sent twenty-seven consecutive attacks towards Kayaba—