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You lose the Buckler's AC bonus if you use your buckler-hand to wield a two-handed weapon.
PRD wrote:
You can also use your shield arm to wield a weapon (whether you are using an off-hand weapon or using your off hand to help wield a two-handed weapon), but you take a -1 penalty on attack rolls while doing so. ... In any case, if you use a weapon in your off hand, you lose the buckler's AC bonus until your next turn.
You also lose the buckler's AC bonus when you use it for a shield bash, or when you use the shield arm to cast a somatic-component spell.