Hi! They are fired with the F key on Keyboard, or on Controller with the 'Face Button Left' (PS: Square, Xbox: X), and you can cycle between them using the G key or D-Pad Left. I have now added this information to the Control Manual, so thanks for asking!
If a both hands are holding weapons that do not have the 'Low Drain' Special Effect (e.g. pistols), then the weapons will regenerate their ammo slower. Equipping an External Capacitor will remove the penalty for weapons held in that hand.
