Warhammer Fantasy Dual Wield Ruling
So 'dual wielding' isn't a term that's used in Warhammer Fantasy's rules. But models frequently have more than one weapon.  As a general rule however, this is more for the ability to choose the right tool for the right job, as opposed to using multiple weapons at the same time.

There are two times when a character will use two different weapons at once:
1) Brace of pistols: Some characters have the option of taking multiple pistols.  They're stat block will explain exactly how they work, since 'repeater pistols' work slightly differently than 'repeater handbows' which are again slightly different from someone who is just covered in flintlock pistols.

2) Models with extra hand weapons can use them in combat for an extra attack.  There are a whole bunch of times where this doesn't work, but basically as long as the model is on foot, and not using any special weapons, or magical weapons, then an extra hand weapon will grant 1 extra attack.

If a model has multiple different kinds of weapons, the general rule is that you choose which one he is using at the start of the round.