Poke around on the bug boards long enough you'd find it. There are a lot of issues involved, not the least of which is it is a coding nightmare to truly fix. From what I've been able to garner, anything that doesn't involve a ton of coding only addresses some of the issues.

For example, if your dual wield is lighter, then when you pick up and rewield the original primary weapon, you'll only have one weapon (since wielding a heavier weapon removes the lighter one rather than automatically moving it to the dual wield slot).

I believe Zulg was looking into having multiple commands to address the issues, but since that never reached fruition I suspect it turned into a bigger nightmare than originally anticipated.

Bottom line is learn to live with the fact that when you are disarmed, your primary wield will be punching and your dual wield stays the same.