True, just settling this into my head straight.
If you learn shield that offers +25 "passive DB" as a weapon, and you have +50 in it, and it's in your off hand (-20). You are +50 with the sword in the primary hand, and +50 in TWC.
(all numbers below OB/DB) all assume 1:1 fight.
If you TWC sword and shield 25/25. You take the off hand penalty to the shield attack, and per TWC the parry DB bonuses no longer stack. 25 (sword) 5 (shield)/25, net 55, with two attack rolls.
If you attack with the sword in the primary hand 25/25 and use the shield passively, it's worth +25 DB vis one foe. 25/50 net 75, with one attack roll.
If you full parry with the Shield and ignore the sword you get +25 from the shield bonus. 0/75, net 75 and one roll.
If you Full parry with the sword and passive the shield you get +5 shield bonus to the sword and +25 DB from the shield. 0/80, net 80 and one roll.
If you full parry with both, you get +5 for the sword and +25 for the shield, the parry DB overlaps per TWC rules so the 50 itself doesn't double. 0/80, net 80 and two rolls.
I think that's right, or did I screw any of that up?