I went back to the old code.
Code:
You are using:
<right hand> (Glowing) an ebony hand crossbow (superb)
[HP:100%] [ME: 62%] [PE:100%]
> wear dagger
You can't wield two weapons at once.
vs.
Code:
> eq
You are using:
<right hand> a throwing dagger (brand new)
[HP:100%] [ME: 62%] [PE:100%]
> wear ebony
You hold an ebony hand crossbow in your hand.
This is a bug. It shouldn't have mattered which weapon was trying to be wielded in your off-hand. It should have told you that you can't wield two weapons at once, since you don't have dual wield.
Some things that should have tipped you off that this was a bug: It says 'hold' not 'wield' for the crossbow. It also doesn't give you a skill message such as "An ebony hand crossbow feels like a part of you!'.
This is still a bug today. I'll have the full fix so that rogues can't wield two weapons as soon as I have some time.