Dual wielding is extremely limited in D2. The easiest way to explain it is - remember the D2 dual wield barb skills? Double Swing, Frenzy, WW - those are the only skills that can ever be true dual wielding skills. You cannot add missiles to any of those skills, which means they will never have good aoe (Windform) or single target and can only be used to apply buffs to your char (Blood Hatred).
Skills like death gaze, iron spiral and noctule were broken since their introduction not only because they didn't proc and grant offensive stats from the off hand, but also because they never actually hit with the off hand despite playing the double attack animation on your character.
So essentially, it's impossible to make good skills by Median standards and have them be true dual wield. This is why dual wield support for the assassin has been completely dropped and on the barbarian it has been shifted to actually provide a benefit (using all melee skills simultaneously). Of course a weapon equipped in your off hand is still a stat stick which provides some, but not all of its, stats so if you feel like that is superior to a shield for your assassin build you are free to go for it.