Is the buckler penalty to attack rolls when using both hands its own special penalty or its armor check penalty?
If I got a mithral/darkwood buckler have no penalty to attack rolls?
It's own special penalty.
Armor Check Penalties are not applied to attack rolls.
The buckler penalizes your attack roll with your off-hand because you have something kind-of big and heavy strapped to that same hand that makes you a little bit awkward when attacking with the hand.
Making the buckler out of special materials does not eliminate this penalty.