12.2.1 - Multiple Weapons
Core Rules, p. 32
If a model is equipped with multiple different Melee type weapons which may be used to make attacks, the model must select which weapon its attacks will be resolved with. A model can only select 1 weapon and all attacks it makes that combat phase are resolved with that weapon. No other weapons they are equipped with can be used that Combat for the models Attacks. Which weapons models in a unit will be using in the Combat are declared at the Beginning of Combat step of the Initiative Order (see 12.1). The selected weapon may modify a models Initiative (I) characteristic.