besides the LB, some jobs work against each other, when you bring 2 of them. in regards of tanks having 2 Dark Knights in the party basically removes the utility a DRK offtank offers, since using The Blackest Night on the DRK maintank is counter productive. i don't know if you can overwrite his shield, but when the MT uses it on himself he gets a 20% shield, if the OT uses it on the MT it's only a 10% shield.