If you combine Fire+Ice resistance in the form of Ice Armor+Fire Shield or Fire Armor+Ice Shield, you should be resistant to both elementals. This is half true.
If an physical attack were to contain fire or ice elemental (Yang's claws for instance), you would take 2x damage.
If you follow my rules on elemental physical weakness, this is still true with character equipment. What's worse is that this behavior extends to the Protect Ring, Adamant Armor, Dragoon armor set, Crystal Helmet, and other equipment and combos that protect against Fire+Ice elementals.
That's something to think about if you are planning on using elemental attacks on enemies in a hack.