(...) An Orc warrior on foot armed with a single choppa adds +1 to his Strength in the first round of combat when he charges. This bonus does not apply if a weapon is carried in each hand, or for Boar, Chariot or monster riders - in such cases the weapon's natural brutishness is cancelled out by the difficulty of wielding it.
The Choppa bonus applies to character models on foot that are armed with a a single ordinary non-magical choppa. It does not apply to individuals armed with magic weapons as they receive the appropriate magical bonuses instead. (...)
Note that the bonus armour save for using a hand weapon and shield does not apply to an Orc wielding a a choppa. Foot models with a choppa in each hand (additional hand weapon) do get normal +1 Attack for having two "hand weapons", though they lose the +1 Strength on the charge.
[p. 6 of AB]