In my games Kundane doesn't give you an extra attack. The only thing special about that sword is the fact that it has a "0" in speedfacter...big deal for a short sword.
And about the chaos blade dual: All you have to do is put 2 stats in shortsword which of course isn't any problem for you lowlevel character since you also need stars in katana and your other weapon to do that "killercombo" of yours. Not to mention the 3 stars in dualwield in order to actually effectively use
And all that in order to kill whatever is vulnerable to the +2 chaos blade by draining their DEX.
It will take you at least 5 hits with your main hand (chaos blade) to even kill the enemies with the lowest
DEX by draining them.
And as, well let's take a Nabassu, has 17 DEX it will require 9 hits with the chaosblade in order to kill it. If you haven't downed it by 9 hits with your main hand...well perhaps you should consider playing a different class.
The only encounters I can see where this form of attack has any value is with mages. I'm not sure if the drain goes through stoneskin and other protections, but if it doesn't, them this tactic is completely useless in my opinion.
Oh, I mentioned 2 stars bacause I figure you meant a fighter/thief, User. I know I wouldn't let my frontliner be a regular thief.
Just my 2 cents anyway.