http://wiki.project1999.com/Skill_Dual_Wield
Basically the higher level your dual wield, the more likely your offhand will swing when the delay is met.
A person with 10 dual wield may swing their offhand 5% of the time whereas a person with 200 dual wield will swing it 80% of the time. (note these numbers are arbitrary as I don't know what the actual values are).
Don't worry about it too much... since you are a Rogue, you will cap your dual wield every level. You will see it swing more and more as you level up.
Also, don't believe anyone who says Dex increases chance to dual wield, it does not.
Edit - STR does increase the speed in which you lvl up dual wield so there's that... but again you'll most likely max it at each level (unless you are being plvled of course) so don't worry too much about it... you'll want Str anyways since you're trying to do the most deeps!