Commanders Bone Plate has 2 "defensive" slots.
It's written (http://castleage.wikia.com/wiki/Forge)
For equipment with multiple slots, the items forged into these slots must be of different types.
For example, if you have Dukes Bone Blade, you could have 1 piercing ingot and 1 elemental offensive gem, or two different elemental offensive gems (fire and earth), but not two of the same (so you couldn't have 2 Physical Piercing or 2 Earth Offense).
So my question is : Has sby try Bronze Ingot of Physical Resistance [+15PhyRes] + Bronze Ingot Stack of Physical Resistance [+25PhyRes] on Commanders Bone Plate ?