If the models in a unit carry two special close combat weapons, they must choose which of the weapons to use at the start of the first round of a close combat (the entire unit must use the same weapon, but characters can always choose separately). Whichever weapon they select must then be used for the entire combat, unless the weapon they are using is somehow destroyed – it's no easy thing to change to a different weapon partway through a fight.
If an infantry model is armed with an additional hand weapon or a brace of pistols, these grant an extra attack. A model can't, however, wield a brace of pistols or additional hand weapon alongside another type of special weapon such as, say, a halberd.